DB2 Basic Manual

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DB2 Basic

Material

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INTRODUCTION TO A RELATIONAL

DATABASE

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What is a Data base Management System.

A Database management system (DBMS) is a software package that manages data storedin databases. You use a DBMS to control and use the information stored on your computersystem more effectively and efficiently than you can with standard file processing. TheDBMS is an additional layer of software between the program and the stored data .Its theDBMS rather than the application program that has a complete picture of the physical andthe logical structure of the data stored.

In a relational system Data is perceived by users as a collection of tables called relations

Operators generate new tables from old ones The rows are referred to as tuples

Columns are referred to as attributes

Each row in a relation is distinct i.e duplicate rows are not allowed

Each relation must have a primary key, an attribute or a combination of attributeswhose value uniquely identifies the rows.

A relation may also contain foreign keys which reference other relations in a database.

Entire information content of the database is represented as explicit data values (asexplicit values in column positions within rows of a table)

DB2

IBMS Database 2 , commonly referred to as DB2, became generally available in the fallof 1983. DB2 was marketed as a relational database management system for the MVSoperating system. It uses tables to represent the data that it manages .

HOW DOES DB2 ORGANIZE AND ACCESS INFORMATION.

Db2 is a database management system. A more complete description is that db2 is arelational database management system that uses the industry standard Structured Query

Language (SQL) as a central component . A relational database system presents allinformation in the form of tables. A table is just a two dimensional array with horizontalrows and vertical columns. The intersection of a row and a column is a value .A valuemay be a text string , a number or nothing (null value).The name relational database comes from the technical term for a table , a relation . Inaddition , the formal names for the elements of a relation are tuple (row) andattribute(column). But these arent the only terms. A row is called a record and a columnis called a field.Although files and tables have similar structures ,they are processed differently . instandard file processing , the unit of processing is a an individual record while in db2 it isan entire table ,which may have one ,many or no rows.

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DB2 ARCHITECTURE

DB2 as perceived by an individual User

SQL

user

VIEW1 VIEW2

Base

Table B1B2 B3 B4

VSAM

Dataset

D1

D2D3 D4

Virtual

Real

Stored


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DB2 OBJECTS

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STORAGE STRUCTURE

The total collection of stored data is divided up into a number of disjoint databases . Each

database is divided into a number of disjoint spaces several table spaces and indexspaces. A space is a dynamically extendable collection of pages ,where a page is a block of

physical storage (it is a unit of i/o i.e the unit transferred between primary and secondarystorage in a physical i/o operation. The pages are given size of either 4k or 32 k. Eachtable space contains one or more stored tables . a stored table is is a physical representationof a base table .

DATABASE:

A Database in DB2 is a collection of logically related objects that is a logical collection

of stored tables that belong together in some way ,together with their associated indexesand the various spaces that contain those tables and indexes . it thus contains a set of tablespaces (each containing one or more stored tables ) together with a set of index spaces(each containing exactly one index). A given stored table and all its associated indexesmust be wholly contained within a single database. Objects are grouped together into thesame database primarily for operational reasons .Tables can be moved from one databaseto another without having any logical impact on user or user programs.

TABLE SPACES:

A table space can be thought of as a logical address space on secondary storage that is use

to hold one or more stored tables (logical because it is typically not just a set of physicallyadjacent areas) . As the amount of data in those table grows (or as the number increases),so storage will be acquired from the appropriate storage group and added to the table spaceaccordingly. One table space can be approx. 64 billion bytes in size and there is effectivelyno limit to the number of table spaces in a database ,nor to the no. of databases .Fundamentally the table space is the storage unit for recovery and re-organization

purposes; i.e. it is the unit that can be recovered via the recover utility . If the table spaceis very large , however , recovery and re-organization could take a very long time.DB2therefore provides the option to partition a large table space into simple , partitioned andsegmented forms.A simple table space can contain more than one stored table , though one is usually the

best option. One reason for having more than one is that stored records for different storedtables can be clustered together in such a way as to improve access times to logicallyrelated data. In particular , certain join queries would then be handled more efficientlysince the I/O operations would be reduced.Partitioned table spaces are intended for stored tables that are sufficiently large. A

partitioned table space thus contains exactly one stored table , partitioned in accordancewith value ranges of a partitioning columns or column combination. Individual partitionsof a partioned table space are independent of each other in the sense that they can beindependently recovered or reorganized.They can also be associated with different storage groups i.e. it is possible to store

different partitions on different devices and thereby spread the table space I/O load.

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SEGMENTED TABLE SPACE:

Like simple table spaces, they can contain any number of stored tables ;unlike simple tablespaces , however, segmented table spaces do not support any kind of cross tableclustering- that is they do not allow records for different stored tables to be interleaved on

a single page. Instead , they keep the tables physically separated.

INDEX SPACE

An index space is to an index what a table space is to a stored table. However, since thecorrespondence between indexes between indexes and index spaces is always one to one,there are no data definition statements for index spaces; instead, the necessary index space

parameters are specified on the corresponding index definition statements. Thus for e.g.there is no create index space; instead the index space is created automatically when thecorresponding index is created. Like Table spaces ,index spaces can be reorganized and

recovered independently.

INDEX

Index in db2 are based on a structure known as B-tree. A b-tree is a multilevel, treestructured index with the property that the tree is always balanced; i.e. all its leaf entries inthe structure are equidistant from the root of the tree, and this property is maintained asnew entries are inserted into the tree and existing entries are deleted. As a result, the index

provides uniform and predictable performance for retrieval operations. A given table canhave any number of associated indexes. Also if the installation follows ourrecommendation that every base table have a primary key, then every stored table will

have at least one index, namely the primary index.To perform an exhaustive search on a given stored table according to a given index, thedata manager will access all records in that stored table in the sequence defined by thatindex and since that sequence may be quite different from the stored tables physicalsequence, a given data page might be accessed many times. It follows that an exhaustivesearch via a physical sequence unless the index concerned is a clustering index for whichthe sequence defined by the index is the same as, or close to, the physical sequence. Theindex is used to control the physical placement of the indexed records. Clustering indexesare extremely important for optimization purpose. The optimizer will try to choose anaccess path that is based on a clustering index, if one is available, and if clusteringsequence is appropriate for the sql under consideration.

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STORAGE GROUPS:

A storage group is a named collection of direct access volumes, all of which are the samedevice type. Each table space and each index space normally has an associated storage

group. When storage is needed for the space or partition , it is taken from the specifiedstorage group. Within each Storage group, spaces and partitions are stored using VSAMlinear data sets .

SYNONYM:Is an alternate name for a base table or view. Each user can assign his or her ownsynonyms for any table or view that was created by some other fully qualified name.

VIEW:

It is a predefined selection of data in base tables. It is a virtual table that does notphysically exist but is processed as a table. It is derived from one or more base tables, orviews or combinations of views and tables. The view definition is stored in the DB2catalog. Changes to the data in the view can change the data in the base table.

CATALOG :

Contains information about every DB2 object that is maintained by it. Whenever a DB2object is created, dropped or altered by any ddl statement or whenever control over the

object is changed by any dcl statement of sql, an entry is made in the appropriate catalogtable. Catalog table can be accessed using sql, they however cannot be updated by auser(but done so by the system).To retrieve information from the DB2 catalog, the select privilege on the catalog isneeded. Some of the catalog tables are :

SYSTABLES

SYSSTOGROUP

SYSTABLESPACE

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COMPONENTS OF DB2

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SYSTEM STRUCTURE:

The major components of DB2 are SYSTEM SERVICES

LOCKING SERVICES

DATABASE SERVICES

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RELATIONAL DATABASE SYSTEM

MVS OPERATING SYSTEM

TSO

Subsystem

IMS

Subsystem

CICS

Subsystem

DB2 Subsystem

RESOURCE

LOCK

MANAGER

Isolate Users

Locking Services

DATABASE

SERVICES

Access Data

Create Objects

DB2 Catalog

SYSTEM

SERVICES

Perform logging

Manage Log

Inventory Manage

Threads


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SYSTEM SERVICES:

This component handles all system wide tasks including connections to other MVSsubsystems.

It also handles system startup and shut down operator communication.

Managing the system log : The system log is a set of predefined disk data sets that areused to record information for recovering user and system data in the event of a failure.When an active log data set becomes full (or on operator command ), DB2 switches toa new data set and copies the old one to an archive log data set on disk or tape. Whenthe active data sets are full, they are recycled. Information regarding the data setsthemselves is recorded in the BOOT STRAP DATA SET (BSDS).

Gathering system wide statistics, performance and accounting information.

LOCKING SERVICES:

Provided by an MVS subsystem called the IMS Resource Lock Manager (IRLM). Despitethe IMS in its name , the IRLM does not really have anything to do with it. It is ageneral purpose lock manager which controls the concurrent access to db2 data.

DATABASE SERVICES:

The primary purpose of the database component is to support the definition , retrieval andupdate of database data- to implement the functions of the sql. The necessary support is

provided by a series of five sub components :

Precompile

Bind

Runtime Supervisor

Data Manager

Buffer Manager

Together these components support the preparation of the application programs forexecution and the subsequent execution of those programs. The functions of the individual

components are as follows:

PRECOMPILER

This component is a preprocessor for the host language. Its function is to analyse the hostlanguage source module stripping out all the sql statements it finds and replaces them bythe host language CALL statements. From the sql it encounters, the precompiler constructsa database request module which becomes the input to the bind component.

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THE PROCESS:

Searches for and expands DB2 related include members .

Searches for sql statement in the body of the programs source code .

Creates a modified version of the source program in which every sql statement in the

program is commented out and a call to the db2 runtime interface module along withapplicable parameters , replaces each original sql statement.

Extracts all sql statements from the program and places them in a dbrm to ensure thatthese two items are inextricably tied.

Reports on the success or failure of the precompile process.

The precompiler searches for sql statements embedded in EXEC SQL and END-EXEC.

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PRECOMPILATION

It uses the SQL statements to build a DBRM for P, which it

stores away as a member of an MVS partitioned data set

DBRM contains a copy of the original SQL statements,

together with additional information

COBOL SOURCE

MODULE P

PRECOMPILER- Source listing

- Diagnostics

- Cross-References

etc.

-

-

-

CALL Long-Interface

--

-

-Modified COBOL

Source Module P.

-

-

-

- DBRM for P

(SQL statements, etc.)


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BIND:

This component is the compiler for the sql statements. It reads the sql statements from theDBRMS and produces a mechanism to access data as directed by the sql statements being

bound. The bind plan accepts as input one or more DBRMs produces from the previousDB2 program precompilation. The output of the bind plan is an application plancontaining executable logic representing optimized access paths to DB2 data. Anapplication plan is executable only with a corresponding module. Before u can run a DB2

program, regardless of the environment, an application plan name must be specified.The major functions of bind are:

Parsing and syntax checking

Optimization

Authorization

Check that DB2 tables and columns being accessed conforms to the correspondingDB2 catalog information.

BIND PLAN vs. BIND PACKAGE:

In some cases however, a DBRM may not be bound directly into a plan. Instead it mayfirst be bound into a package and then finally, the packages may be bound into a plan.There are certain disadvantages of directly binding the DBRMs into an application plan :

If an individual DBRM needed to be recompiled for any reason(for eg some index wasdropped),the entire plan had to be recompiled and rebound.

If multiple plans involved the same DBRM , that same DBRM had to be compiledmultiple times and , if that DBRM ever needed to be recompiled , then all relevant

plans had to be recompiled and rebound in their entirety.

Adding a new DBRM to an existing plan required (again) a recompilation and rebindof the entire plan.

Partly as a consequence of the foregoing points, bind and rebind times are becomingunacceptably high in some DB2 installations and availability was suffering as a result.

The package concept was introduced to remedy these deficiencies. If a given DBRMneeds to be recompiled, all that has to be done is an appropriate package bind it is notnecessary to recompile the entire plan. Indeed, it may not be necessary to do a new plan

bind to incorporate the new package either.

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RUNTIME SUPERVISOR:

The runtime supervisor is resident in main memory when the application program is

executing. Its job is to oversee that execution. When the application program requestssome database operation to be performed (wishes to execute some sql), control first goesto the runtime supervisor, which uses control information in the application plan to requestthe appropriate operations on part of the data manager.

DATA MANAGER:

The data manager can be thought of as a very sophisticated access method. It performs allof the normal access method functioning such as search, retrieval, update , index

maintenance etc. Broadly speaking, the data manager is the component that manages thephysical database(s). It invokes other system components as necessary in order to performdetailed functions such as locking, logging, I/O operations etc during the performance ofits basic task.

BUFFER MANAGER:

The buffer manager is the component responsible for physically transferring data betweenexternal storage and virtual memory. It employs sophisticated techniques to get the best

performance out of the buffer pools under its care and to minimize the amount of physicalI/O actually performed.

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DB2 Application Programs Preparation & Execution :-

(Control Flow Diagram Overview)

Linkage

Editor

Modified

Source

Module

Object

Module

Compiler

DBRM

Appln.

Plan

BIND

(Load Module)

application plan

Runtime

Supervisor

Data

Manager

Buffer

Manager

Load

Module

DB

Source

Module

Precompiler


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DATATYPE FUNCTIONS

AND

OPERATORS

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INTRODUCTION:

The basic data object is the scalar value ;the object appearing at the intersection of a givenrow and a given column of a given table. Each scalar value is of some scalar data type. Foreach such data type, there is also an associated format for writing literals of that type.Scalar objects can be operated upon by means of certain scalar operators. DB2 also

provides certain scalar functions which can be regarded as scalar operators. Scalar objectsand operators can be combined to form scalar expressions.

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DB2 DATATYPES

DATA TYPE DESCRIPTION VALURANGE STORAGE

REQUIRED

INTEGER Full word

binary integer 1 - 2 billion 4 bytes

SMALL INT Half word

binary integer 1-32767 2 bytes

DECIMAL(p,q) p-total no. of 0


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DB2 DATATYPES

DATA TYPE DESCRIPTION STORAGE RANGE

REQUIRED VALUE

VARCHAR(n) Variable length 0


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CONSTANTS & LITERALS

Integer : Signed or unsigned decimal

integer with no decimal point

Eg : 4 -15 +364 0

Decimal : Signed or unsigned decimal

number with decimal point

Eg : 40 -95.7 +364.05 0.0007

Float : Written as a decimal constant,followed by a character E,

followed by an integer constant

Eg : 4E3 -95.7E46 +364E.5 07E1

Character : Written either as a string of characters

string enclosed in a single quotes or a string of pairs

hexadecimal digits enclosed in single quotespreceded by letter X.

Eg : 123MAIN St.

PIGON

X FIF2F3F40D156247

Graphic: : Written as a string of double byte characters

string preceded by the character the whole enclosed in single quotes and

preceded by letter G.

Eg : G


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NUMERIC OPERATORS:

ARE : +,-,*,/

FUNCTIONS:

CONCATENATION ( ): can be used to concatenate two character strings or twographic strings. E.g (INITIALS || LASTNAME)

CHAR: Converts a date, time ,timestamp to its character string representation.

DATE: Converts a scalar value to a date.

DAY: Extracts the day portion of a date or a timestamp.

DAYS: Converts a date or timestamp to a number of days.

DECIMAL: Converts a number to decimal representation.

DIGITS: Converts a number(decimal or integer) to a character string representation.

FLOAT: Converts a number to a floating point representation.

HEX: Converts a scalar value to a character string representing a internal HEX code

HOUR: Extracts the hours portion of a time or timestamp.

INTEGER: Converts a number to integer representation.

LENGTH: Computes the length of the scalar value in bytes .

MICROSECOND: Extracts the microsecond portion of a timestamp.

MINUTE: Extracts the minute portion of a date or timestamp.

SECOND: Extracts the second portion of a date or timestamp.

SUBSTR: Extracts a part of the string from a string.Assume = AARTIe.g. SUBSTR(SNAME,1,3) would extracts AAR

TIME: Converts a scalar value to time.

TIMESTAMP: Converts either (a) Single scalar value or(b) a pair of scalar values (representing

a date and time resp.) to a timestamp

VALUE: Converts a null value into a non null value.e.g. Select custno,fname,lname,value((H):||homeph,(w):|| workph, no phone)

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VARGRAPHIC: Converts a character sting to a graphic string.

YEAR: Extracts the year portion of a date or timestamp.

SPECIAL REGISTERS

Each individual user is assigned an authorization ID. That id is used to sign on to thesystem, and serves as the primary id for the user in question. Tables and other objects thatare purely private to that user will typically be created under the control of, and hence beowned by the primary id.

Each functional area(eg each department) in the organization is also assigned anauthorization id. However, that id is typically not given the sign-on authority; users signon to the system under their primary id. Once signed on, users can operate under their

primary id or using the sql statement (SET CURRENT SQLID) they can switch to asecondary id. An external subsystem such as IBMs RACF keeps track of the secondaryid(s) that can legitimately be used by a given primary id. A given primary id can have anynumber of secondary ids also that the same secondary id can be used by any number of

primary ids.

SCALAR EXPRESSIONS

Six types of scalar expression are

Numeric

Character String

Graphic String

Date

Time

Timestamp expressions

Assignments :Assignment operations are performed when values are retrieved from the DataBase orstored into the DataBase.

Comparisons :The general form of a comparison is

Comparand operator comparand where

Two comparands must be compatible

The operator is any of the following> , = ,


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Examples

WEIGHT * 454=1000SUBSTR (PNAME,1,1) = CFINAL_DATE = REVIEW DATE - CURRENT DATESUM (QTY) =500

NULLS :

Means missing information. The problem with missing information is that is frequentlyencountered in the real world. For example , historical records sometimes include suchentries as date of birth not known:; meeting agendas often show a speaker as to beannounced and police records may have entries as present whereabouts not known.Hence it is desirable to have some way of dealing with such situations in our formaldatabase systems. SQL systems like DB2 represent missing information by means of

special markers called NULLS. For e.g. we might say loosely that the weight of some partis null. What we mean by such a statement is that (a) the part exists (b) of course , it has aweight but (c) we do not know what that weight is. Instead we mark that slot as null.In general any column can contain nulls unless the definition of that column explicitlyspecifies NOT NULL. If a given column is allowed to contain nulls, and a row is insertedinto the table and no value is provided for that column, DB2 will automatically place a nullin that position.In DB2, a column that can accept nulls is physically represented in the stored database by

two columns, the data column itself and a hidden indicator column ,one byte wide ,that isstored as a prefix to the actual data column.

NOT NULL WITH DEFAULT means that the column in question cannot contain NULLS,

but that it is nevertheless still legal to omit a value for the column on insert. If a row isinserted and no value is provided for some column on which not null with defaultapplies,DB2 automatically places one of the following default values in that position:

Zero for numeric columns

Blanks for fixed length columns

Empty (zero-length string) for varying length string columns.

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EMBEDDED SQL

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EMBEDDED SQL:

Any SQL statement that can be used at the terminal can also be used in an applicationprogram. There are various differences of detail between a given interactive SQLstatement and its corresponding embedded form.

Embedded SQL statements are prefixed by EXEC SQL and are terminated by END-EXEC

SQL statements can include references to host variables

Any tables (base tables or views) used in the program must be declared

After any SQL statement has been executed, feedback information is returned to theprogram in an area called the SQL Communication Area (SQLCA)

The embedded SQL SELECT statement requires an INTO clause specifying the hostvariables.

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HOST VARIABLES

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HOST VARIABLES:Host variables are variables declared in the working storage section and are used by DB2when it moves data between your program and a table. They are defined according to the

rules of programming language.Host variables can appear in SQL data manipulation statements. They are generally usedfor designating a target for retrieval. Such variables can appear in the following positions :

Into clause in select or fetch (target for retrieval)

Where or having clause (value to be compared)

Set clause in update (value to be assigned)

Values clause in insert (value tobe inserted)

Example

EXEC SQL

Select status , city into :status , :cityfrom employeewhere sno = :given-sno;

END-EXEC

SQL statements can include references to host variables, such references are prefixed witha colon to distinguish them from SQL column names

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INDICATOR VARIABLES

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INDICATOR VARIABLE

In general if there is a chance that the source or a retrieval operation might be null, the usershould include an INDICATOR VARIABLE in the into clause in addition to the targetvariable as illustrated in the following example:

EXAMPLE :

EXEC SQL

Select status , cityInto :status:status-ind, :city:cityindFrom sWhere sno = :Given-Sno

END-EXEC.

For processing and finding out whether the fields were NULL or not, a check can beperformed as :

IF STATUS-IND < 0 THEN /*STATUS WAS NULL */

Indicator variables can be used in the VALUES clause to insert NULL VALUES.

EXAMPLE: -

IF COLORIND < 0 OR CITYIND < 0

EXEC SQL

INSERT INTO P (PNO,COLOR, CITY)VALUES(:PNO,:PCOLOR:COLOR-IND, :PCITY:CITYIND)

END-EXEC.

Each indicator variable is a half-word integer (pic S9(4) comp).

If defined as an array (occurs clause) ,then it may be used for a list of columns with thefirst occurrence of the indicator variable corresponding to the first column in that listusing a single indicator variable, Nulls can be handled for all the fields of the table.

The indicator variable will have a negative value if the select returns a null value.

If the indicator variable returns a value > 0, it indicates a truncated variable is thelength of the character string before truncation

If the Indicator variables contain :

A negative No. indicates that the column has been set to Null

The value 2 indicates that the column has been set to Null as a result ofData Conversion Error

A positive or a zero value indicates that the column is not Null

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If a column, defined as a char data type,is truncated on retrieval because the hostvariable is not large enough, the indicator Variable, contains the original length of thetruncated column

EXAMPLE: -

01 DEPT-INDICATORS.10 DEPT-IND OCCURS 5 TIMES PIC S9(4) COMP.

EXEC SQLSELECT DEPTNO, DNAME, MGR, HO, LOC

INTO :DCLDEPT: DEPT-INDFROM DEPTWHERE DEPTNO = A00

END-EXEC

Indicator variables can appear on the right hand side of an assignment in the SETclause to set a value to NULL.

EXAMPLE: -

IF RANKIND = < 0

EXEC SQLUPDATE SSET STATUS = :STATUS:STATUS-INDWHERE CITY =LONDON

END-EXEC.

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CURSORS

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CURSORS:

A mechanism allowing an application program to retrieve a set of rows. The cursor facilityallows a COBOL program to gain addressability to individual row occurrences of a many-rows result table

Following steps to be performed to use cursors

Declare cursor : to define cursor

Open cursor : to create result table

Fetch cursor : to retrieve rows from cursor, one at a time to beexecuted in a loop

Close cursor : to close the cursor

Declaring (defining) a cursor is done in the data division of your program.This is purely declarative in nature. Therefore no information is being retrieved from thedatabase yet. The Cursor name should begin with a letter and must not exceed 18characters.When the open statement is encountered the select in the cursor declaration is executed.The open cursor statement not only executes the selection of data from the DB2 database

but also establishes the initial position of the cursor in the results table. One program canhave multiple cursors.

Declaring a CursorEXEC SQL

DECLARE cursor CURSOR FOR SELECT col1, col2.... FROM tableWHERE condition [FOR UPDATE OF col1, col2 ..]

END-EXEC

Opening a CursorEXEC SQL

OPEN cursorEND-EXEC

Fetching a row from cursor :EXEC SQL

FETCH cursor INTO : host var1, :host var2,...END-EXEC.

Closing a cursor :EXEC SQL

CLOSE cursorEND-EXEC.

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Example-Based on a Single Table

Operations Involving Cursors :-

EXEC SQLDECLARE X CURSOR FORSELECT S#, SNAME, STATUS

FROM S WHERE CITY = :YEND-EXEC.

EXEC SQLOPEN X

END-EXEC.

LOOP UNTIL NO MORE ROWS OR ERROR

EXEC SQLFETCH X INTO :S#,:SNAME.:SNAME-IND

END-EXEC.

PROCESSING STATEMENTS .

EXEC-SQLCLOSE X

END-EXEC.

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DATA MODIFICATION:

Often an application program must read data and then based on its value, either update ordelete data. One can use the UPDATE or DELETE SQL statements to modify and delete

rows from DB2 tables. These statements are similar to select statements which operate ona set of data at any given point of time.

This is accomplished with a cursor and a special clause of the update and delet statementsusable only by embedded SQL namely :WHERE CURRENT OF. The cursor is declaredwith a special FOR UPDATE OF CLAUSE.

A for update ofclause appears with:

Select statement to indicate what columns can be updated when retrieved

The columns to be updated must be listed in the for update of clause or the declare You do not have to select a column to update it

Rules for Update

The select statement must be on a single table and not on a join

If the declare cursor statement contains a subquery it must not be on the same table asthe main query

You cannot use distinct, group by, order by or built-in functions

Only those columns are eligible for updation which are selected in the for.. updateclause

The current forms of UPDATE & DELETE

EXEC SQLUPDATE tableSET field = : exp [, field = : exp]WHERE CURRENT OF cursor

END-EXEC.

EXEC SQLDELETE FROM tableWHERE CURRENT OF cursor

END-EXEC.

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Example: (Use of Cursor)

EXEC SQL

Declare C1 cursor forSelect Deptno, Deptname, Mgrno From DeptWhere ADMRDEPT = :ADMRDEPTfor update of MGRNO

END-EXEC.

PROCEDURE DIVISION.

MOVE A00 TO ADMRDEPT.

EXEC SQL

OPEN C1END-EXEC.

PERFORM 200-MODIFY-DEPT-INFO UNTIL NO-MORE-ROWS.

EXEC SQL

CLOSE C1END-EXEC.

200-Modify-Dept-Info.

EXEC SQL

Fetch C1 into :deptno, : deptname, : mgrnoEND-EXEC.

If sqlcode < 0GO TO 9999-error-paragraph.

If sqlcode = +100Move NO to more-rows

Else

EXEC SQLUpdate Dept Set MGRNO = 00000Where current of C1

END-EXEC.

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DATA RETRIEVAL

Pseudocode for retrieving data from an application join ,

using cursors:EXEC SQL

Declare Deptcur cursor forSelect Deptno, DeptnameFrom Dept

END-EXEC.

EXEC SQL

Declare Empcur cursor forSelect empno, salary from emp

where workdept = :hv-workdeptEND-EXEC.

EXEC SQL

Open DeptcurEND-EXEC.

Loop until no more dept rows or error

EXEC SQL

Fetch deptcur into :Deptno , :Deptname

END-EXEC.

Move deptno to HV-WORKDEPT

EXEC SQL

Open EmpcurEND-EXEC.

Loop until no more employee rows or error

EXEC SQL

Fetch empcur into:empno, :salary

END-EXEC.

Process retrieved dataEnd Loop (2)End of Loop (1)

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WITH HOLD OPTION:

This is an optional specification on a cursor declaration. The significance can beunderstood by considering what happens in its absence.

Suppose we need to process some large table, one row at a time by means of a curso andupdate a few of the as we go. It is often desirable to divide the work up into batches and tomake the processing of each batch into a separate transaction (by issuing a separatecommit at the end of each one); thus, e.g. a table of one million rows might be processed

by a sequence of 10,000 transactions, each one dealing with just 100 rows. This way , fore.g. if it becomes necessary to roll a given transaction back, then at most 100 updates willhave to be undone, instead of potentially as many as a million.

The problem with this approach ,however is that every time we issue a commit, weimplicitly close the cursor, thereby losing our position within the table. The first thing eachtransaction has to do, therefore is to execute some re-positioning code in order to get backto the row that is due to be processed next. And that re-positioning code can often be quitecomplex, especially if the processing sequence is determined by a combination of severalcolumns.

If the cursor declaration specifies with hold, however, commit does not close the cursor,instead, leaves it open, positioned such that the next FETCH will move it to the next rowin sequence. The possibly complex code for repositioning is thus no longer required.

However it is important to note that the first operation on the cursor following the commitmust be fetch. Update and delete current are illegal.

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ERROR / EXCEPTION

HANDLING

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WHENEVER:

The whenever statement has the syntax :

EXEC SQL

WHENEVER condition actionEND-EXEC.

Where condition is one of the following :

NOT FOUND means SQLCODE = 100

SQLWARNING means SQLCODE >0 & NOT = 100

SQLERROR means SQLCODE < 0

ACTION specifies a CONTINUE or a GO TO statement.

The whenever statement causes the program to automatically check the sqlcode. Basedon the value it finds, it takes the action you specify. Some application programmers prefer(for structured reasons) to avoid the whenever statement, and use all specific errorchecking after each SQL statement is issued.If you omit the action for a whenever statement, the default of continue will apply for thatcondition.There is no limit to the no. of whenever statements you can use.

Example using WHENEVER

1000-INQUIRY.

EXEC SQLWHENEVER SQLERRORGOTO 1000-UNDO

END-EXEC.

EXEC SQL

SELECT FLD1, FLD2 INTO : FLD1, :FLD2FROM EMP-TABLE WHERE CODE = 113

END-EXEC.

1000-UNDO

DISPLAY ERROR! CANT PROCEEDEXEC SQL

ROLLBACKEND-EXEC.

1000-EXIT.

EXIT.

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TRANSACTION PROCESSING

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WHAT IS A TRANSACTION ?

A transaction is a logical unit of work. Consider the following example. Suppose for thesake of example the parts table contains an additional column totqty representing the totalquantity for the part in question. The value of the totqty for any part is supposed to beequal to the sum of all sp.qty values taken all over sp rows for that part. Now consider thefollowing sequence of operations, the intent of which is to add a new shipment(S5,P1,1000) to the database:

EXEC SQL

WHENEVER SQLERROR GOTO UNDOEND-EXEC.

EXEC SQL

INSERT INTO SP VALUES (S5,P1,1000)END-EXEC.

EXEC SQL

UPDATE PSET TOTQTY = TOTQTY + 1000WHERE P# = P1

END-EXEC

EXEC SQL

COMMIT WORKEND-EXEC.

UNDO:EXEC SQL

ROLLBACKEND-EXEC

FINISH:

RETURN

The insert adds the new shipment to the sp table, the update updates the totqty column forthe part p1 appropriately.

The point of example is that what is presumably intended to be a single atomic transactioncreate a new shipment infact involves 2 updates to the database. What is more is thatthe database is not consistent even between thise two updates. Thus a logical unit of workis not necessarily just one SQL operation ; rather it is a sequence of several suchoperations, in general, that transforms a consistent state of the database into another

consistent state, without necessarily preserving consistency at all intermediate points.Now it is clear that what must not be allowed to happen in the example is for one of the

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two updates to executed and the other not (because then the database would be in aninconsistent state). Ideally we would want that both the updates are done, but we cannothave such a guarantee: there is always a chance that things will go wrong. For example asystem crash may occur between the two updates.

But a system that supports transaction processing does provide the next best thing to sucha guarantee. Specifically, it guarantees that if the transaction executes some updates andthen a failure occurs, for whatever reason before the transaction reaches its normaltermination, the those updates will be undone. Thus the transaction either executes in itsentirety or is totally cancelled.

The system component that provides this atomicity is know as the transaction manager andthe COMMIT WORK and ROLLBACK WORK are the keys to the way it works.

The COMMIT WORK operation signals successful end-of-transaction :it tells thattransaction manager that a logical unit of work has been successfully completed.

The ROLLBACK operation by constrast, signals unsuccessful end-of-transactionindicating an inconsistent state and all the updates made by the logical unit of work must

be rolled back or undone.

SYNTAX:

COMMIT

The SQL commit statement takes the formEXEC SQL

COMMIT [WORK]END-EXEC.

ROLLBACK

The SQL ROLLBACK statement takes the form

EXEC SQL

ROLLBACK [WORK]END-EXEC

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CONCURRENCY AND LOCKING

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THREE CONCURRENCY PROBLEMS:

DB2 is a shared system; i.e. it is a system that allows any number of transactions to access

the same database at the same time. Any such system requires some kind of concurrencycontrol mechanism to ensure that current transactions do not interfere with each othersoperation and DB2 includes such a mechanism namely locking.There are essentially three ways in which things can go wrong- three ways. They are

The lost update problem

The uncommited dependency problem

The incorrect analysis problemThe lost update problem:

Consider the situation as shown in the figure:

Transaction A retrieves some row R at time t1 ; transaction B retrieves the same row attime t2; transaction A updates the row(on the basis of the values seen at time t1) at time t3;and transaction B updates the same row (on the basis of the value seen at time t2, whichare the same as seen at time t1) at time t4. Transaction As update is lost at time t4,

becauses transaction B overwrites it without even looking at it.

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The Lost Update Problem

TRANSACTION A TIME TRANSACTION B

- -

- -

FETCH R T1 -

- -

- T2 -

- FETCH R

UPDATE R T3 -

- -

- T4 UPDATE R

-


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The uncommitted dependency problem:

This problem arises if one transaction is allowed to retrieve (or update) a row that has beenupdated by another transaction and has not yet been committed by that other transaction.For if it has not been committed, there is always a possibility that it never will becommitted but will be rolled back instead- in which case the first transaction will have datathat now no longer exists. Consider the figure given below:

In this example , transaction A sees an uncommitted update at time t2. That update is thenundone at time t3. Transaction A is therefore operating on false assumption that row R hasthe value as seen at time t2, where as in fact , it has whatever value it had prior to time t1.As a result, transaction A may well produce incorrect output.

Inconsistent analysis problem:

This problem could be a simple summation problem.

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The Uncommitted Dependency Problem

TRANSACTION A TIME TRANSACTION B

- -

T1 UPDATE R

-

-

- -

FETCH R T2 -

- -

- -

- T3 ROLLBACK

- -TRAN A BECOMES DEPENDENT ON AN UNCOMMITTED CHANGE

AT TIME T2


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How DB2 solves these problems:

DB2s concurrency control mechanism is based on a technique called locking. The basicidea of locking is simple: when a transaction needs and assurance that some of the object itis interested in typically a database row - will not change in some unpredictable mannerwhile its back is turned , it acquires a lock on that object and there by to prevent them fromchanging it. The first transaction is thus able to carry out its processing in the certainknowledge that the object in question will remain in stable state for as long as thattransaction wishes it to

The two types of locks that can be placed are shared lock(S) and exclusive lock (X). We

assume that if a transaction requests a lock that is currently not available , the transactionsimply waits until it is. In practice, the installation can specify a maximum wait time ; thenif any transaction ever reaches this threshold in waiting for a lock, it times out and thelock request fails ( a negative SQLCODE is returned)

From the compatibility matrix , two inferences can be drawn:

If transaction A holds an X lock on row R , then a request from transaction B for a lock ofeither type on R will cause B to go into a wait state. B will wait until As lock is released.

If transaction A hold a shared lock (S) lock on row R , thenA request from transaction B for an X lock on R will cause B to go into a waitstate (and B will wait until As lock is released);

A request from transaction B for an S lock on R will be granted (that is , B willnow also hold an S lock on R)

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A

B X S

X N N

S N Y

COMPATIBILITY MATRIX


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Transaction requests for row locks are always implicit. When a transaction successfullyfetches a row , it automatically acquires an S lock on that row. When a transactionsuccessfully updates a row, it automatically acquires and X lock on that row. If it alreadyholds an S lock on that row,as it will in FETCH/UPDATE or FETCH/DELETESEQUENCE, then the update or delete promotes the S lock to X level.

The lost update problem revisited

The above figure shows what would happen to the interleaved execution under the lockingmechanism of DB2. As one can see, transaction As update at time t3 is not accepted

because it is an implicit request for an X lock on R, and such a request conflicts with the Slock already held by transaction B; so A goes into a wait state. For analogous reasons , Bgoes into a wait state at time t4. Now both transaction are unable to proceed, so there is noquestion of any update being lost.DB2 thus solves the lost update problem by reducing itto another problem but at least it does solve the original problem. This problem is calledthe deadlock problem, discussed later.

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Handling the Lost Update Problem

TRAN A TIME TRAN B

- -

FETCH R T1 -

(acquires S lock on R) - -

- -

- T2 FETCH R- (acquires S lock On R)

-

UPDATE R T3 -

(request X lock on R) --

Wait

Wait UPDATE RT4 (request for X lock on R)

WaitWait


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The uncommitted dependency problem revisited

Transaction As operation at time t2 is not accepted , because it is an implicit request for alock on R, and such a request conflicts with the X lock already held by B; so A goes into await state and remains so until B reaches a synchpoint(either commit or rollback), whenBs lock is released and A is able to proceed; and at that point A sees a committed value(either the pre B value, if B terminates and with a ROLLBACK , or the post b valueotherwise. Either way, A is no longer dependent on an uncommitted update.

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Handling the Uncommitted Dependency Problem

TRAN A TIME TRAN B

- -

- T1 UPDATE R

- (X lock on R)

- -

FETCH R -(request for S lock on R) T2 -

-

Wait

`

Wait SYNCPOINT

T3 (Release X lock

Wait on R)

resume : FETCH R T4

(X lock on R)`


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Deadlocks .A 'deadlock' occurs when program A locks page X exclusively and attempts to

lock page Y, while program B has already locked page Y exclusively and attempts to lockpage X. Neither program can continue without the required lock. DB2 cancels one of theprocesses (the one with the fewest log records) with a time out code. To avoid thissituation, you can:

Use the same sequence of update. Both programs should advance along thetables in ascending key order; this makes it less likely that they will cross eachother's paths and attempt to lock the same page. Both programs should accessvarious tables in the same sequence, for the same reason.

Avoid clashing updates through different paths (if possible). The problemwith such updates can be exemplified as follows:User A is updating page 1 using index X for access to the data;User B is updating page 2 using index Y for access to the data.

However, each index also references the data on the other page (i.e., Index Xreferences data on page 2 and index Y references data on page 1).

Therefore, after the data is reached by means of one index, and after that data isupdated, each query must modify the other index so it would reflect the new,updated data. However, that other index is still locked by the other query.

Such deadlocks are hard to avoid. You may watch out for some conditionswhich make this situation more likely:

Large number of indexes on a table, with frequent deletes orupdates.

Multi-row updates and deletes.

Large value used for SUBPAGES for the index.

Use frequent COMMITs. The chance that a page needed byprogram A will already be held by program B is thereby reduced.

Use cursor with FOR UPDATE OF ... This technique locks with INTENTUPDATE, ahead of time, all the pages which you will need. This is a morerelaxed lock than an exclusive lock, but it still prevents other programs fromlocking one of your pages exclusively. Thus, less contention, fewer deadlocksand fewer time outs will occur.

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Explicit locking facilities

In addition to the implicit locking mechanism described earlier, DB2 provides certainexplicit facilities which the programmer should be aware of. The explicit facilitiesconsists of : the SQL statement LOCK TABLE , the ISOLATION PARAMETER on the

bind plan command, the table space LOCKSIZE parameter and the ACQUIRE/RELEASEPARAMETER on the bind plan.

LOCK TABLE :

Depending on the versions, this command either locks up the table or an entire table space

SYNTAX

LOCK TABLE table IN mode MODE

Where mode is SHARE or EXCLUSIVE & table must designate a base table not aview.Once a lock is acquired no other transaction will be able to access any part of the table inany way - until the original lock is released. When that original lock is released depends on

the RELEASE parameter on BIND.The Lock table statement can be used to control locks from within a DB2 application

program. Every individual page lock uses system resources in storage and processing timeA single table lock reduces the storage and processing time required by the many smalllocks. This results in saving of system resources.

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ISOLATION PARAMETER:

Specifies the isolation level for the application plan being bound.DB2 supports twoisolation levels for every transaction

Cursor Stability (CS)

Repeatable Read (RR)

These are used while cursor manipulation

Cursor Stability

DB2 takes a lock on the page the cursor is accessing and releases the lock on that page,

when the cursor moves onto a different page(This is not done when we use FOR UPDATE of statement in cursor)The lock on the last page is released at commit time (or at Thread de-allocation time)

Repeatable Read

In cursor stability, while your transaction reads data, other transaction could change thedata you have already read. In repeatable read, DB2 holds all page locks while the cursoris moving all till the transaction commits (or till thread deallocation).Cursor Stability provides more /higher concurrency while repeatable read provide moreconsistency.

The problem with CS is that a transaction operating at that level may have a recordchanged behind its back as in inconsistent analysis problem and so may produce awrong answer.

By contrast, a transaction that operates under isolation level RR, can behave completely as

if it were executing in a single user system.

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LOCKSIZE PARAMETER:

Physically, DB2 locks data in terms of pages or tables or table spaces, depending on whatwas specified as the LOCKSIZE for the relevant table space in the CREATE or ALTERTABLESPACE operation.

For a given table space, the LOCKSIZE can be specified as PAGE, TABLE, TABLESPACE or ANY.

TABLESPACE : means all locks acquired on data in the tablespace will be at thetablespace level

TABLE : means locks acquired on data in the tablespace will be at the table level

PAGE : means locks acquired on data in the tablespace will be at the page levelwhenever possible.

ANY : (Which is the default) means that DB2 itself will decide the appropriatephysical unit of locking for the tablespace for each plan .Defaults to a page lock, but ifthe no. of pages that are locked exceeds an installation default, DB2 does a lockescalation and automatically locks a larger unit.

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ACQUIRE/RELEASE PARAMETERS

DB2 always implicitly acquires locks of some kind

The acquire and release parameters on the BIND command specify when such tablespacelevel locks are to be acquired and released .For ACQUIRE, the possible specifications are

USE & ALLOCATE;For RELEASE, they are

COMMIT & DEALLOCATE

While Binding, DB2 allows us to specify transaction level lock acquiring and releasingparameters.

ACQUIRE:

ACQUIRE (use) - This option tells DB2 to take locks at the time of SQL statementsexecution. This is DB2 default.

ACQUIRE (allocate) - This option tells DB2 to take all necessary locks at the start of thetransaction.

RELEASE:

Similar to the DB2 acquire parameters, there are 2 release parameters.

RELEASE (commit) - This option tells DB2 to release all locks of transaction committime. This is the DB2 default.Release (de-allocate) - This option tells DB2 to release all locks only when the programends (i.e. the thread is de-allocated).

All combinations except ACQUIRE (Allocate) and RELEASE (commit) are allowed.

For more concurrency, use ACQUIRE (use) and RELEASE (commit).This is the DB2 default.

For better performance, use ACQUIRE (ALLOCATE) and RELEASE(DEALLOCATE).

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- 52 -

Fields of Sql Communication Area

01SQLCA.05 SQLCAID PIC X(8).

05 SQLCABC PIC S9(9) COMP-4.

05 SQLCODE PIC S9(9) COMP-4

05 SQLERRM.

49 SQLERRML PIC S9(4) COMP-4.

49 SQLERRMCPIC X(70).

05 SQLERRP PIC X(8).

05 SQLERRD OCCURS 6 TIMES PIC S9(9) COMP-4.

05 SQLWARN.

10 SQLWARN0 PIC X.

10 SQLWARN1 PIC X.

10 SQLWARN2 PIC X.

10 SQLWARN3 PIC X.10 SQLWARN4 PIC X.

10 SQLWARN5 PIC X.

10 SQLWARN6 PIC X.

10 SQLWARN7 PIC X.

05 SQLEXT PIC X(8).


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SQL WARNINGS

SQLWARN0 - This indicates that any of the following warnings

are flagged.

SQLWARN1 - This is flagged if a value is truncated when

assigned to a host variable

SQLWARN2 - This is set to W if some of the rows werent

considered by a column function because the

column being processed contained null values

SQLWARN3 - Here a W means you did not supply enough host

variables in an into clause to match the columns in

a select clause

SQLWARN4 - This flag is set to W if a dynamic SQL update or

delete statement does not have a Where clause

SQLWARN5 - This indicates that the SQL statement is valid only

in SQL/DS and not in DB2

SQLWARN6 - This indicates a W when an arithmetic operation

produces an unusual date or timestamp


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SQLERRD is an array of six full word items.The third of the six, contains

useful information.

After an insert,delete or update statement, sqlerrd(3) contains the no.

rows the statement affected.

Eg.

EXEC SQL

DELETE FROM EMP

WHERE DNO IN

(SELECT DNO FROM MASTER_DEPT)

END-EXEC.

Display SQLERRD(3) ROWS WERE DELETED.

SQL WARNINGS


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DB2 ERROR CODES

SQLCODE Keyword Meaning+100 SELECT Row not found during FETCH,

SELECT, UPDATE or DELETE

+304 Program Value and host variable are

incompatible

-305 Variables Null value occurred, and no indicator

variable was defined

-501 Cursor Cursor named in FETCH or CLOSE is

not open-551 Authority You lack the authority to access the

named object, possibly because its

name is not spelled correctly

-803 Updating Duplicate keys not allowed

-805 Plan DBRM not bound into this plan

-811 SELECT Embedded SELECT or sub select

returned more than one row


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DATA DEFINITION

In this topic we will understand the two database objects and the SQL commands used in

creating, modifying or removing their definitions from the database:

TABLE

CREATE TABLE

ALTER TABLE

DROP TABLE

INDEX

CREATE INDEX

ALTER INDEX

DROP INDEX

TABLES

A table is a collection of rows having the same attribute or columns. In other words it is amatrix where the intersection of every row and a column is a value , which is the smallestunit of data that can be retrieved or changed. A row is a smallest unit of data that can beinserted or deleted.

The structure of a table in a tablespace can be created with the following command:

CREATE TABLE TABLE_NAME

(COLUMN1 DATATYPE {NOT NULL|NOT NULL WITH DEFAULT} ,

COLUMN2 DATATYPE {NOT NULL|NOT NULL WITH DEFAULT } ,

COLUMN3 DATATYPE {NOT NULL|NOT NULL WITH DEFAULT } ,

.

.

PRIMARY KEY (COLUMN1,COLUMN2,...)

FOREIGN KEY RELATION_NAME(COLUMN3,COLUMN4,...)

REFERENCES TABLESPACE_NAME.TABLE_NAME

ON DELETE {RESTRICT|CASCADE|SET NULL})IN DATABASE_NAME.TABLESPACE_NAME

A table name can be simple or fully qualified. A simple table name has to begin with acharacter and cannot be more than 18 characters long. Except for the first character thename can contain any alphanumeric character including the special characters (-,@,#,$). Afully qualified table name is a simple table name prefixed by the owners ID . For e.g. ifthe user MKTG is creating a table CUSTOMER then the table name will beMKTG.CUSTOMER.e.g..

1> simple table CUSTOMER can be created with the following command;

CREATE TABLE CUSTOMER

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(CUSTID SMALLINT NOT NULL WITH DEFAULT,

CNAME CHAR(15) NOT NULL WITH DEFAULT,

CITY CHAR(15) NOT NULL WITH DEFAULT,

PRIMARY KEY (CUSTID)

IN MKTGDATA.TABLSPC1

This command creates the CUSTOMER table which is having the CUSTID field as itsprimary key. The NOT null with default clause prevents the column from having nullvalues and specifies a default value to be used if data for the specific column is missing.The default values of a column depend on the datatype of the column. The table belowshows the default values different datatypes in DB2.

2>Table ORDER which is a child table of CUSTOMER and PARTS tables can be createdafter creating both the parent tables, with the following command:

CREATE TABLE ORDERTAB

(ORDID SMALLINT NOT NULL WITH DEFAULT,ORDERDT DATE NOT NULL WITH DEFAULT,

RATE DEC(8,2)NOT NULL WITH DEFAULT,

CUSTID SMALLINT NOT NULL WITH DEFAULT,

PARTTID SMALLINT NOT NULL WITH DEFAULT,

PRIMARY KEY (ORDID,CUSTID,PARTID),

FOREIGN KEY FKEY1(CUSTID) REFERENCES CUSTOMER

ON DELETE RESTRICT

FOREIGN KEY FKEY2(PARTID) REFERENCES PARTS

ON DELETE RESTRICT

IN MKTGDATA.TABLSPC1

The above command creates the table ORDER with ORDID, CUSTID and PARTID asthe primary keys. These keys ensure uniqueness of the record within the table. The foreignkey FKEY1 ensures that the CUSTID field in this table consists of only one of thosevalues which are existing in the primary key field of that table which appears in thereferences clause i.e. CUSTOMER . While The foreign key FKEY2 ensures that thePARTID field in this table consists of only one of those values which are existing in the

primary key field of that table which appears in the references clause i.e. PARTS.

Tables once created can be altered with the following command:

ALTER TABLE TABLE_NAME

[COLUMN1 DATATYPE {NOT NULL|NOT NULL WITH DEFAULT} ,]

[AUDIT (NONE|CHANGES|ALL)]

[PRIMARY KEY (COLUMN1,COLUMN2,...)]

[FOREIGN KEY RELATION_NAME(COLUMN3,COLUMN4,...)

REFERENCES TABLESPACE_NAME.TABLE_NAME]

[DROP PRIMARY KEY]

[DROP FOREIGN KEY (FOREIGN_KEY_NAME)]

e.g...

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1>In the CUSTOMER table CREDITLIMIT field can be added with the followingcommand:

ALTER TABLE CUSTOMER

ADD CREDITLIMIT INTEGER NOT NULL

2> The primary key definition of the ORDER table can be modified in the following way:

ALTER TABLE ORDERTAB

ADD PARTNO SMALLINT NOT NULL

DROP PRIMARY KEY

PRIMARY KEY (PARTNO)

INDEXES

They are used for efficient access to data . It is an ordered set of pointers to the data in aDB2 table. The rows in a table are stored in a particular order . But when the data has to beaccessed, the entire table needs to be scanned due to which the performance of the systemwould be affected. Here indexes can prove very significant as they provide alternate meansof access to the data thereby decreasing the I/O and the processing time. A user cannot byany means tell the system when to use or when not use the indexes. It is only the DB2optimizer which makes the choice at BIND time.

An index is a physical entity which can be based on one or more columns of a table andcan be created any time after the table is built. There is no limit on the number of indexeswhich can be built on a table , but a partitioned table must have atleast one index . This iscalled as a PARTITIONING INDEX and is used to define the scope of each partition andthereby assign the rows of the table to their respective partitions.

The second type of index is a CLUSTER INDEX . When a cluster index is created on atable the rows of that table are physically stored in the order of those fields which have

been mentioned in the cluster index. The order of these rows can also be specified asascending or descending. There can be only one cluster index per table. If no such index iscreated, it loads the data in order of the first index field defined.

UNIQUE INDEXES in DB2 can be created to disallow the user from entering duplicate

values into a field constrained by the unique index. For e.g... the primary key field of atable must have unique values in all the rows, this relational datamodel concept is wellsupported by a Unique Index on the primary key field.

Indexes can be created with the following command :

CREATE [UNIQUE] INDEX INDEX_NAME

ON TABLE_NAME (INDEX_COLUMN_NAME [ASC|DESC] [,...])

[CLUSTER [(PART INTEGER VALUES (CONSTANT OR RANGE))]]

[SUBPAGES (1|2|4|8|16) ]

[BUFFERPOOL (BP0|BP1|BP2)]

e.g.

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The following command creates a unique clustering index CUSTIDX on the fieldCUSTID of the CUSTOMER table:

CREATE UNIQUE INDEX CUSTIDX

ON CUSTOMER (CUSTID)

USING STOGROUP MKTGSUBPAGES 8

CLUSTER

Each indexpage physically consists of 4k bytes, which can be further divided into amaximum of 16 subpages . The subpage is a logical unit of locking an index. Thereforewhen a subpage is used ,smaller amount of storage is locked for individual update. Thusfewer users are kept waiting at a given point of time and this causes increase inconcurrency . The SUBPAGE clause defines the number of subpages into which one index

page can be divided. The CLUSTER clause can be used to create clustering index, whichis the fastest path for sequential access.

Definition of an index can be altered by the following command:

ALTER INDEX INDEX_NAME

[PART (INTEGER)]

BUFFERPOOL(BP1|BP1|BP2)]

INDEXSPACE

As table data is stored in a tablespace, index data is physically stored in indexspace. Thisindexspace contains an ESDS for every index . If the indexspace is partitioned ,then each

partition has its own indexspace. It is divided in pages of the size 4k and further thesepages are divided into 1 to 16 units which are called subpages. This subpage as discussedearlier is the unit of locking.Apart from this, unlike a tablespace an indexspace need not be explicitly defined as it getscreated automatically whenever an index is created.

Exercise

1. Create the following tables

SALESMAN CUSTOMER PARTS ORDER

Slid Custid Partid OrdidName Cname Description OrderDateRegion City Act_Rate Rate

Min_Stock UnitsPartidCustid

Slid

2. Create primary key indexes for all the above created tables so as to enforce

uniqueness for these keys.

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3. Describe the different types of indexes and their advantages with the create indexcommand

4. Add a new numeric field credit limit in the customer table with the alter command.This field should have a default values and cannot be left null.

5. What will be the effect of the following drop table command on the other objects in

the database which are related to the table ORDERS.Drop Table Orders.

RETRIEVAL OPERATIONS

In this topic we will discuss SQL statements for performing the following operations onthe data described below :

SELECT

INSERT

UPDATE

DELETE

SAMPLE DATABASE OF ABC Inc.

CUSTOMER TABLE

CUSTID CNAME CITY

1005 Spectrum Mumbai

1004 Lloyds Madras

1001 Pentasys Delhi

1002 Hertz Mumbai1003 Procter Pune

PARTS TABLE

PARTID DESCRIPTION ACT_RATE MIN_STOCK

101 Nuts 4.00 50000

106 Bolts 2.50 15000

104 Nails 5.00 12500

105 Latches 15.00

103 Hinges 6.00 12000

102 Handle 15.00 8500

ORDERTAB TABLE

ORDID ORDERDATE RATE UNITS PARTID CUSTID

201 O1/12/96 3.00 300 101 1001

202 12/11/95 5.50 250 103 1001206 06/22/96 4.50 375 104 1002

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204 04/1896 3.75 175 101 1004

205 05/05/96 13.50 235 102 1003

203 11/0896 5.50 180 103 1003

SELECT STATEMENT

SQLgives the SELECT statement to retrieve the data from the database.This statement hasdifferent clauses attached to it which have been explained further. Unlike the other SQLstatements INSERT, UPDATE, DELETE this statement can work on multiple tables atthe same time.The basic syntax of the SELECT statement

SELECT {ALL /DISTINCT/ scalar-expression/s}

FROM table/s

[ WHERE conditional-expressions]

[ GROUP BY column/s]

[ HAVING conditional-expressions]

[ ORDER BY column/s ]

Let us now proceed with the different types of SELECT statements :

5.1.1 SIMPLE SELECT

To retrieve all the columns of the CUSTOMER table the following SELECT statementwill have to be executed:

SELECT * FROM CUSTOMER

RESULT(screen dump)

The above result indicates that the statement has retrieved all the records of theCUSTOMER table and all the fields too. Whenever a * is given by default all thecolumns re selected in the sequence in which they have been defined while creating thetable. If one has to retrieve only specific column the SELECT statement can be modifiedas below :

SELECT CNAME, CITY

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FROM CUSTOMER

RESULT(screen dump)

SELECT ELIMINATING UNIQUE / DISTINCT VALUESIf only unique values have to be retrieved from the database the word DISTINCT should be specified before the column list. For e.g. if different CITY values from theCUSTOMER tables have to be retrieved the following SELECT statement can help :

SELECT DISTINCT CITYFROM CUSTOMER

RESULT(screen dump)

Without DISTINCT i.e. by default the above SELECT statement would retrieve the sameCITY multiple times i.e. ALL

COMPUTED VALUES RETRIEVAL

Many a times it happens that the data which is in the database has to represented in adifferent way. For e.g. if one has to retrieve the actual value(RATE * QTY) of every partsold to every individual customer from the PART_CUST table the statement will be:

SELECT PARTID,CUSTID, RATE*UNITSFROM ORDERTAB

RESULT(screen dump)

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Computed retrieval might also be required when an expression has to be used on any ofthe field or fields while retrieving . For e.g. If the DESCRIPTION value of the part has to

be in upper case while the value is stored in lower/mixed case the following SELECTstatement is valid:

SELECT PARTID, UPPER(DESCRIPTION)FROM PARTS

CONDITIONAL RETRIEVAL

For retrieving data of specific criteria or multiple criterias which can be combinedthrough one SELECT statement, the statement has to use a WHERE condition. Someexamples below will explain the different ways of using WHERE.

E.g 1

SELECT CNAME, CITYFROM CUSTOMERWHERE CITY = Mumbai

screen dump

E.g 2

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SELECT ORDID, ORDERDTFROM ORDERTABWHERE CUSTID = 1001 AND

UNITS > 200

screen dump

The above SELECT statement shows the data satisfying multiple conditions on the

TABLE ORDERTAB.

E.g 3

SELECT *FROM PARTSWHERE ACT_RATE < 4 OR

MIN_STOCK > 10000

screen dump

The above results shows that those records which are satisfying any one of the twoconditions are selected.

The WHERE condition can have different operators to compare the value of the fields. thenumeric operators (, = , ,>=,


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The BETWEEN condition is used to define a range for comparing numeric values.It can be considered as a shortcut rather than involving two conditions with AND.

SELECT *

FROM PARTSWHERE MIN_STOCK BETWEEN 10000 AND 20000

screen dump

The same result as shown in the above screen can also be obtained by the followingquery :

SELECT *FROM PARTSWHERE MIN_STOCK > 10000 ANDMIN_STOCK < 20000

With the BETWEEN operator a NOT condition will retrieve exactly the opposite data asshown below :

SELECT *FROM PARTSWHERE MIN_STOCK NOT 10000 AND 20000

IN

The IN operator is an alternative for the OR used for individual conditions. when afield has to be compared against a set of given values the IN operator is useful.

SELECT *FROM ORDERTAB

WHERE PARTID IN (101, 104)

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screen dump

The above result can also be obtained without an IN operator with the following SELECTstatement :

SELECT *FROM ORDERTABWHERE PARTID = 101 OR

PARTID = 104

screen dump

The NOT IN operator is used for deselecting the above data and selecting the data whosePARTID value is anything except 101, 104.

SELECT *FROM ORDERTABWHERE PARTID NOT IN (101, 104)

screen dump

LIKE

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LIKE operators are used for comparing a value which either begins with certaincharacters, or ends with certain characters or contains certain characters. the followingsexamples deal with different ways of using the LIKE operator

SELECT DESCRIPTIONFROM PARTSWHERE DESCRIPTION LIKE N%

screen dump

The result shows only those PART records which have a name starting with the alphabetN.

SELECT DESCRIPTIONFROM PARTSWHERE DESCRIPTION LIKE %S

This command will retrieve only those records from the PART table which has the lastalphabet as S irrespective of what the other alphabets in the DESCRIPTION field.

As we observed in the above examples, a % stands for any characters. Similarly _(underscore) stands for a single character. This can be used to search a particularcharacter in a string at a particular position.

SELECT *FROM CUSTOMERWHERE CNAME LIKE _e%

screen dump

This SELECT statement retrieves only those records which have the alphabet e in itssecond position of the CNAME field.

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Using a NOT LIKE in place of LIKE in the last SELECT statement will retrieve all theother records from the CUSTOMER table except those displayed in the above screendump.

CHECKING FOR NULL VALUES

For retrieving values which contain a NULL the operator IS has to be used. Forchecking a NULL value, this is the only valid operator as any other operator like >, NULL

OR

WHERE MIN_STOCK < NULL

OR

WHERE MIN_STOCK = NULL

If one need to retrieve the records having MIN_STOCK value as NULL, the SELECTstatement should be as follows :

SELECT *FROM PARTSWHERE MIN_STOCK IS NULL

screen dump

To check the values which are not NULL, the word NOT can be used with the IS operatoras shown below :

SELECT *FROM PARTSWHERE MIN_STOCK IS NOT NULL

screen dump

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JOINS

A SELECT statement has the ability to select from multiple tables at once. So thisfeature is called as a JOIN whereby two tables are selected through the same statement. Inother words they are JOINS.

DB2 offers different types of JOINS as explained below :

Simple Join

According to Relational Database concepts, a column is responsible for creating arelationship between two tables. As in our sample database the PARTID column relatesthe ORDER table to the PARTS table. Similarly the CUSTID column relates the ORDERtable to the CUSTOMER table. A JOIN can be used for selecting data from any of thesetables at the same time through one SELECT.

SELECT CNAME, RATE, UNITSFROM CUSTOMER, ORDERTABWHERE CUSTOMER.CUSTID = ORDERTAB.CUSTID

screen dump

With the above query, one gets the records belonging to the CUSTOMER table as well asthe ORDER table. The CUSTID field being the common fields in these two tables needsto be compared, while selections from these tables have to take place. It is actually the

WHERE condition which compares the data of these two tables having the same values intheir CUSTID fields. It is necessary that this field has the same datatype and datawidth in

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both the tables, but the name of the field may differ in the two tables. Apart from this thestatement may also have some more WHERE conditions as explained below :

SELECT CNAME, RATE, UNITSFROM CUSTOMER, ORDERTAB

WHERE CUSTOMER.CUSTID = ORDERTAB.CUSTID AND CITY = Mumbai

screen dump

Both these statements above are checking for the equal value of CUSTID fields that theycan be termed as EQUIJOINS. When one SELECT statement retrieves the data from the

multiple tables, each pair of table should be bound by a JOIN between them. In short, theno. of JOINs in a SELECT statement are one less than the number of tables being selectedfrom.

Self Join

As one can select from two tables through one SELECT statement, it is alsopossible to select one table twice in the same SELECT statement. Here is an examplewhich finds all the CUSTOMER records belonging to the same CITY

SELECT A.CNAME, B.CNAME, A.CITY

FROM CUSTOMER A, CUSTOMER BWHERE A.CITY = B.CITY

screen dump

In the above SELECT it appears as though two different tables A, B are being selectedfrom. In reality A and B are nothing but aliases used for the same table CUSTOMER.This means the first selection of the CUSTOMER table is given a name A while the

second selection is given a name B.

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We also observe in our screen dump that a value like Spectrum is selected twice fromselection A as well as selection B because each value shows up once in each alias and the

predicate of these values is symmetrical. the other records like Hertz, Procter are alsorepeated because of the same reason. To avoid this, one has to add a WHERE conditionwhich checks for non matching predicates as shown in the following SELECT statement :

SELECT A.CNAME, B.CNAME, A.CITYFROM CUSTOMER A, CUSTOMER BWHERE A.CITY = B.CITYAND A.CNAME< B.CNAME

screen dump

SUB-QUERIES

A SELECT statement can also appear as a clause of another SELECT statement.Such SELECT statement are called NESTED queries. The inner queries returns a valuewhich is compared with a value of the outer query. In other words the outer query isexecuted after the inner query. Such type of nesting can be done upto multiple levels.

The example below selects the CNAME and the CITY for those CUSTOMERS who haveordered for PART 103. Here the outer query selects from the CUSTOMER table as thedata to be displayed belongs to the same table. While the inner query finds the CUSTIDvalue from the ORDER table where the PARTID value is 103 :

SELECT CNAME, CITYFROM CUSTOMERWHERE CUSTID IN (SELECT CUSTID

FROM ORDERTAB

WHERE PARTID = 103)

screen dump

The difference in NESTED queries and JOINS is that one needs to give a JOIN only if thedata to be extracted belongs to multiple tables. Nested queries are generally used when the

data belongs to one table while the condition has to be checked in some other table.

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Performance-wise JOINS are preferred as NESTED queries execution is slower thanJOINS (according to IBM Manuals ).

In our example the CUSTID is common between the two tables (ORDER, CUSTOMER).It is this fields value which is checked between the two queries.

The next example discusses nesting of queries upto multiple levels. Irrespective of thenumber of levels to which nesting is done, the execution of the queries remain the same. Inother words, it is always the innermost query which will be executed first and then theenclosing query and so on.

To select the details of the PARTS which are ordered by the CUSTOMER Hertz, thefollowing NESTED query can be given :

SELECT *FROM PARTS

WHERE PARTID IN (SELECT PARTIDFROM ORDERTAB

WHERE CUSTID = (SELECT CUSTIDFROM CUSTOMER

WHERE CNAME = Hertz))

screen dump

CORRELATED SUB QUERY

A Correlated sub query is the one whose value depends upon a variable whichreceives a value from some outer query. Such sub-query has to be repeatedly executed forevery new value of the variable. These types of queries can be used as an alternatesolutions for JOINS.

The following example selects all the CNAME values from the CUSTOMER table whohave ordered for PART 103.

SELECT CNAMEFROM CUSTOMERWHERE 103 IN (SELECT PARTID

FROM ORDERTABWHERE CUSTID = CUSTOMER.CUSTID)

screen dump

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Execution of such correlated queries take place in the following way :

1. The currently selected row from the outer query is stored.2. The sub-query is performed where a field from the row stored in first step is used forselecting specific rows from the table mentioned in this query (Outer reference).

3. The value from the outer query is evaluated on the basis of the results of the sub-queryexecution taken place in step 2. After this evaluation whether the record from theouter query should be selected or no is determined.

4. The same three steps mentioned above are performed for the next row of the outerquery till all the rows of the table are evaluated.

CORRELATED QUERIES ON THE SAME TABLE

Correlated sub- queries can also be fired on the same table as the outer queriestable. These types of queries are useful in selecting data from a single table which isevaluated against some other data of the same table.

This example selects all the details from the PART table whose ACT_RATE ishigher than the ACT_RATE of the PARTS Nails.

SELECT *FROM PARTSWHERE PARTID IN (SELECT PARTID

FROM PARTS

WHERE DESCRIPTION = Nails)

screen dump

OTHER OPERATOR (EXISTS, ANY/ SOME, ALL )

EXISTS

This operator produces a Boolean results. Unlike the other operators where a valueis used, it takes only a sub-query as an argument. The sub-query is evaluated to true orfalse and only if the result is true, the outer query which has the EXISTS operator

produces certain output. The query given below extracts data from the PARTS table onlyif there are more than one part having MIN_STOCK value > 5000.

SELECT *FROM PARTS

WHERE EXISTS(SELECT *

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FROM PARTSWHERE MIN_STOCK > 5000)

screen dump

ANY/SOME

The operators ANY and SOME are interchangeable as the result produced bythem is just the same. The operator ANY / SOME takes all the values produced by thesub-query and evaluates to true if ANY of these values are equal to the value of certainfield of the record selected in the outer query. It is necessary to select a field of the samedatatype as the one against which it is being compared in the main query.

This example selects all the records from the CUSTOMER table whose CUSTID appearsin the ORDER table.

SELECT *FROM CUSTOMERWHERE CUSTID = ANY

(SELECT CUSTIDFROM ORDERTAB)

ALL

As the ANY operator checks for the OR condition , the ALL operator can be checkingthe AND operator. In other words, it checks for each and every value which is selected bythe inner query for satisfying a condition on a field of the outer query.

The query below selects all those ORDERS where UNITS ordered are greater than all theUNITS ordered by the CUSTOMER 1003.

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SELECT *FROM ORDERTABWHERE UNITS> ALL

(SELECT UNITS

FROM ORDERTABWHERE CUSTID = 1003)

screen dump

The above statement examines UNITS value of all the ORDERS placed by CUSTOMER1003. It then find out the ORDERS having UNITS more than the highest UNITS ofCUSTOMER 1003 i.e. 235.

The NOT operator can be attached to all the operators discussed above as in the operatorssuch as IN, LIKE, BETWEEN discussed in the sub- section of 5.1.

AGGREGATE FUNCTIONS

Aggregate functions are those which return a single value like MINIMUM,MAXIMUM, AVERAGE, SUM after evaluating multiple values of a certain column inthe table. These functions appear in the column list of the SELECT statement. The list

below gives the names of the functions and the value returned by it.AGGREGATE FUNCTIONS

FUNCTION DATA TYPE RETURNED

MAX NUMBER/CHAR Largest value in the columnMIN NUMBER/CHAR Least value in the column

COUNT NUMBER/CHAR Number of values in the columnSUM NUMBER/CHAR Sum of values in the columnAVG NUMBER/CHAR AVG of values in the column

These functions can also be used in nested sub-queries or in combination with distinctClause.

Some examples :

1. To find the total no. of CUSTOMERS SELECT COUNT (*) FROM CUSTOMER.2. To find the TOTAL UNITS to ORDER by CUSTOMER 1003 SELECT SUM

(UNITS ) FROM ORDER WHERE CUSTID = 1003.3. To find the ORDER WHERE no. of UNITS order is less than the Avg. .

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WHERE UNITS < (SELECT AVG (UNITS)FROM ORDERTAB

4. To SELECT the no. of CUSTOMERS who have placed ORDERS SELECT COUNT(DISTINCT CUSTID) FROM ORDERTAB

GROUP BY HAVING

This is an optional clause of the SELECT statement. It groups the rows of a tablesurrounding to the value in the column appearing on which GROUPING is done. It has anoptional sub-clause HAVING which checks a condition on every group created byGROUP BY. Only those groups which satisfy the condition are passed to the columnfunction further. One can understand HAVING a filter on the group as the clause,WHERE acts as a filter on the final data being selected.

Group by clause allows only column functions or the those columns on which grouping isdone to be selected in the column list of the SELECT statement. The same is applicable tothe HAVING clause.

This example selects the maximum UNITS value for every CUSTOMER

SELECT MAX (UNITS), CUSTIDFROM ORDERTABGROUP BY CUSTID

screen dump

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In the above example the execution of the query takes place in the following manner :

The GROUP BY clause puts all records which have the same CUSTID value together andcreates on set, the next set has records of the next CUSTID so on it continues to create thegroup till all CUSTIDs are considered. The result of which is shown below . Then the

maximum no. of units are selected from each group.

screen dumpof order by custid on order

We observe in the above result that a CUSTID who has placed a single order (1004) isplaced in a separate group. Let us assume the result is not supposed to show any max valuefor such groups having a single record. In such a case attaching the HAVING clause to theGROUP BY clause is necessary. Thus the previous query changes to -

SELECT MAX (UNITS), CUSTID

FROM ORDERGROUP BY CUSTIDHAVING COUNT (*)>1

ORDER BY

The final result being displayed by the SELECT statement can be sequenced in a particularway by using the ORDER BY clause. The data which is selected is displayed either in theorder of its selection or in the order of its physical storage. When this order has to be seton a certain field of the table one can use the ORDER BY clause.

In the statement below, we arrange all the records in the order of ORDERDATE field inthe ORDER table.


ORDER BY ORDERDT

screen dump

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One can also order the data on multiple fields. The fields have to appear in the order byclause in the order of the required sequence. If the above e.g. one has to show the recordsor ORDER table in the sequence of CUSTID and within every single value of CUSTID,the records should be sequenced on the ORDERDATE field the query given will be

SELECT *FROM ORDERTABORDER BY CUSTID, ORDERDT

UNION

The UNION operators is used when records appearing in either or both the tablesreferred in the SELECT statement are to be retrieved. The operation is embedded in twoSELECT statements and both these SELECT statements must

1. Have the same no. of columns

2.3. Have the columns which belong to same datatype

the example below selects the PARTID value which have a MIN_STOCK above 10000 orthose parts which are ordered by

CUSTOMER 1001SELECT PARTIDFROM PARTSWHERE MIN_STOCK >10000UNION

SELECT PARTIDFROM ORDERTAB

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WHERE CUSTID = 1001

screen dump

The result indicates that a PARTID selected only once. The UNION operator implicitlyeliminates the duplicate value unless an ALL is used with it as shown below :

SELECT PARTIDFROM PARTSWHERE MIN_STOCK >10000UNION ALLSELECT PARTIDFROM ORDERTABWHERE CUSTID = 1001

screen dump

AGGREGATE FUNCTIONS

MAX NUMBER/CHAR - Largest value in the columnMIN NUMBER/CHAR - Least value in the columnCOUNT NUMBER/CHAR - Numb

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