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Structured Query Language

Many database management systems support some version of structured query language (SQL).

In some DBMSs (i.e., ORACLE) SQL is the primary data manipulation interface. Consequently,

SQL is a very important topic. The purpose of this document is to introduce you to the majorSQL statements and to show you how they work. This document will concentrate primarily on

ORACLE SQL; however, some attention also will be given to other versions of SQL. This is not

a complete reference of SQL. If you are interested in a more detailed coverage I can suggest

several good textbooks for outside reading.

SQL commands can be broken into the 3 following functional groups: .

- Data Definition Language (DDL) - used to define the schema (structure) of the

database

- CREATE TABLE

- ALTER TABLE- CREATE INDEX

- DROP TABLE

- DROP INDEX

- Data Manipulation Language (DML) - used to retrieve and

update from the database

- SELECT

- INSERT

- UPDATE

- DELETE

- Data Control Language (DCL) - used to manipulate the processing of data and erform

other misc. functions

- COMMIT

- ROLLBACK

I. SQL DATA DEFINITION (DDL)

TABLES

CREATE TABLE - Define the structure of a new table

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Format:

CREATE TABLEtablename ({col-name type [(size)][constraint],...});

The 'constraint' clause in the CREATE TABLE statement is used to enforce referential integrity.

Specifically, PRIMARY KEY, FOREIGN KEY, and CHECK integrity can be set when youdefine the table. The syntax for key and check constraints is shown below.

{PRIMARY KEY | FOREIGN KEY} (local-field) [REFERENCESforeign-field] }

for attribute constraints...

CHECK (condition)

Example: Define the student table

CREATE TABLE STUDENT( STUID CHAR(5),

LNAME CHAR(10) NOT NULL,

FNAME CHAR(8),

MAJOR CHAR(7) CHECK (MAJOR IN (HIST,ART,CIS)),

CREDITS INTEGER CHECK (CREDITS > 0),

PRIMARY KEY (STUID));

ALTER TABLE - Add a column to the "right" of an existing table, modify an existing attribute,

or drop an existing column or constraint.

Format:

ALTER TABLEtablename

{ADD {col-name type [(size)] | constraint}}

| MODIFY {col_name type [(size)]

| DROP {col-name [drop-clause];

where the drop clause is ...

DROP {PRIMARY KEY | UNIQUE {col-name} | CONSTRAINT constraint|

Example1: Add column called MINOR to STUDENT table

ALTER TABLE STUDENT ADD MINOR CHAR(8);

Example2: Drop the MGR-SSN column

ALTER TABLE EMPLOYEE DROP MGR-SSN;

Example3: Modify existing attribute

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ALTER TABLE DEPT MODIFY MINOR CHAR(10);

LIMITATIONS/ENHANCEMENTS:

When you add a column, all existing tuples get the extra column filled with NULL values. You

have to go in and update the column to enter valid data later to get rid of the NULLs. You can

only add or drop a single column at a time in the ALTER statement.

CREATE INDEX - Create an index on attribute(s) within a table

Indexes are used to improve system performance by providing a more efficient means of

accessing selected attributes.

Format:

CREATE [UNIQUE] INDEX index-name ONtable-name {(col-name [ASC |

DESC])};

Example1: Create an index on the STUID attribute of the STUDENT table

CREATE INDEX stuindex ON STUDENT (STUID);

Example2: Create a unique index on SSN of EMPLOYEES and make it sort in reverse order

CREATE UNIQUE INDEX empindex ON EMPLOYEES (SSN) DESC;

Example 3: Create a composite index on COURSENUM and SDUID from ENROLL table

CREATE INDEX enroll-idx ON ENROLL (COURSENUM, STUID);

DROP TABLE - Remove a table (and all data) or an index on a table from database. If the table

has any foreign key constraints then the CASCADE CONSTRAINTS clause is required to dropthe table.

Format:

DROP TABLEtable-name [CASCADE CONSTRAINTS];

DROP INDEX index-name;

Example1: Delete the table STUDENT (no foreign key constraints)

DROP TABLE STUDENT;

Example2: Delete the table ENROLL (with foreign key constraints)

DROP TABLE ENROLL CASCADE CONSTRAINTS;

Example3: Remove the emp-name index on employee table

DROP INDEX emp-name;

When you drop a table you also delete all data currently in that table. Be careful!

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II. SQL DATA MANIPULATION LANGUAGE (DML)

The DML component of SQL is the part that is used to query and update the tables (once they

are built via DDL commands or other means).

By far, the most commonly used DML statement is the SELECT. It combines a range of

functionality into one complex command.

SELECT

Used primarily to retrieve data from the database. Also used to create copies of tables, create

views, and to specify rows for updating.

General Format: Generic overview applicable to most commercial SQL implementations - lots of

potential combinations. There are several variations available in Oracle.

SELECT {field-list | * | ALL | DISTINCT | expression}

FROM table-list

WHERE expression

GROUP BY group-fields

HAVING group-expression

ORDER BY field-list;

Only the SELECT and the FROM clauses are required. The others are optional.

FROM - A required clause that lists the tables that the select works on. You can define

"alias" names with this clause to speed up query input and to allow recursive "self-joins".

WHERE - An optional clause that selects rows that meet the stated condition. A "sub-

select" can appear as the expression of a where clause. This is called a "nested select".

GROUP BY - An optional clause that groups rows according to the values in one or

more columns and sorts the results in ascending order (unless otherwise specified). The

duplicate rows are not eliminated, rather they are consolidated into one row. This is

similar to a control break in traditional programming.

HAVING - An optional clause that is used with GROUP BY. It selects from the rows

that result from applying the GROUP BY clause. This works the same as the WHEREclause, except that it only applies to the output of GROUP BY.

ORDER BY - An optional clause that sorts the final result of the SELECT into either

ascending or descending order on one or more named columns.

There can be complex interaction between the WHERE, GROUP BY, and HAVING clauses.

When all three are present the WHERE is done first, the GROUP BY is done second, and the

HAVING is done last.

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Example 1: Select all employees from the 'ACCT department.

SELECT * FROM EMPLOYEES WHERE EMP-DEPT = 'ACCT';

Example 2: Show what salary would be if each employee recieved a 10% raise.

SELECT LNAME, SALARY AS CURRENT, SALARY * 1.1 AS PROPOSEDFROM EMPLOYEES;

SIMPLE SINGLE TABLE RETRIEVAL

Example 1: Retrieve all information about students ('*' means all attributes)

SELECT * FROM STUDENT;

STUID LNAME FNAME MAJOR CREDITS

S1001 Smith Tom History 90

S1010 Burns Edward Art 63S1015 Jones Mary Math 42

S1002 Chin Ann Math 36

S1020 Rivera Jane CIS 15

S1013 McCarchy Owen Math 9

Example 2: Find the last name, ID, and credits of all students

SELECT LNAME, STUID, CREDITS

FROM STUDENT;

LNAME STUID CREDITS

Smith S1001 90

Burns S1010 63

Jones S1015 42

Chin S1002 36

Rivera S1020 15

McCarthy S1013 9

Example 3: Find all information about students who are math majors

SELECT *FROM STUDENT

WHERE MAJOR = 'Math';

STUID LNAME FNAME MAJOR CREDITS

S1015 Jones Mary Math 42

S1002 Chin Ann Math 36

S1013 McCarthy Owen Math 9

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Example 4: Find the student ID of all History

SELECT STUID

FROM STUDENTWHERE MAJOR = 'History';

STUID

S1001

Example 5: Retrieve a list of all majors that currently have students

SELECT DISTINCT MAJOR

FROM STUDENT;

MAJORArt

CIS

History

Math

MORE COMPLEX SINGLE TABLE RETRIEVAL

The WHERE clause can be enhanced to be more selective. Operators that can appear in

WHERE conditions include:

=, ,< ,> ,>= , 30;

STUID

S1015

S1002

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Example 2: Find the student ID and last name of students with between 30 and 60 hours

(inclusive).

SELECT STUID, LNAME

FROM STUDENT

WHERE CREDITS BETWEEN 30 AND 60;

this is the same as...

SELECT STUID, LNAME

FROM STUDENT

WHERE (CREDITS >= 30) AND (CREDITS

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Example 4: Retrieve the ID and course number of all students without a grade in a class.

SELECT STUID, COURSENUM

FROM ENROLL

WHERE GRADE IS NULL;

STUID COURSENUM

S1010 ART103A

S1010 MTH103C

NOTE: IS NULL may only appear in the WHERE clause. Also note that you say "IS NULL",

not "= NULL". NULL means "unknown" and does not really have a value in the normal sense.

Example 5: List the ID and course number for all students that successfully completed classes

(the inverse of #4 above).

SELECT STUID, COURSENUMFROM ENROLL

WHERE GRADE IS NOT NULL;

STUID COURSENUM

S1001 ART103A

S1020 CIS201A

S1002 CIS201A

S1002 ART103A

S1020 MTH101B

S1001 HST205A

S1002 MTH103C

Example 6: List the course number and faculty ID for all math courses.

SELECT COURSENUM, FACID

FROM CLASS

WHERE COURSENUM LIKE 'MTH%';

COURSENUM FACID

MTH101B F110

MTH103C F110

NOTE: % is a wildcard for any number of characters. _ is a wildcard that replaces a single

character. They can be used together along with normal characters.

COLUMN FUNCTIONS (AGGREGATE FUNCTIONS)

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Aggregate functions allow you to calculate values based upon all data in an attribute of a table.

The SQL aggregate functions are: Max, Min, Avg, Sum, Count,StdDev, Variance. Note that

AVG and SUM work only with numeric values and both exclude NULL values from the

calculations.

Example 1: How many students are there?

SELECT COUNT(*)

FROM STUDENT;

COUNT(*)

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NOTE: COUNT can be used in two ways. COUNT(*) is used to count the number of tuples that

satisfy a query. COUNT with DISTINCT is used to count the number of unique values in anamed column.

Example 2: Find the number of departments that have faculty in them.

SELECT COUNT(DISTINCT DEPT)

FROM FACULTY;

COUNT(DISTINCT)

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Example 3: Find the average number of credits for students who major in math.

SELECT AVG(CREDITS)

FROM STUDENT

WHERE MAJOR = 'Math';

AVG(CREDITS)

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ORDERING OF THE QUERY RESULT

The ORDER BY clause is used to force the query result to be sorted based on one or more

column values. You can select either ascending or descending sort for each named column.

Example 1: List the names and IDs of all faculty members arranged in alphabetical order.

SELECT FACID, FACNAME

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FROM FACULTY

ORDER BY FACNAME;

FACID FACNAME

F101 Adams

F110 ByrneF221 Smith

F202 Smith

F105 Tanaka

Example 2: List names and IDs of faculty members. The primary sort is the name and the

secondary sort is by descending ID (for seniority purposes).

SELECT FACID, FACNAME

FROM FACULTY

ORDER BY FACNAME, FACID DESC;

GROUPING QUERY RESULTS

The GROUP BY clause is used to specify one or more fields that are to be used for organizing

tuples into groups. Rows that have the same value(s) are grouped together.

The only fields that can be displayed are the ones used for grouping and ones derived using

column functions. The column function is applied to a group of tuples instead to the entire table.

If the HAVING clause is used, it takes the intermediate table produced by the GROUP BY and

applies further selection criteria. Note that the data are aggregated when the HAVING is

applied, so the HAVING expression should be written appropriately.

Example 1: Find the number of students enrolled in each course. Display the course number and

the count.

SELECT COURSENUM, COUNT(*)

FROM ENROLL

GROUP BY COURSENUM;

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COURSENUM COUNT(*)

ART103A 3

CIS201A 2

HST205A 1

MTH101B 1MTH103C 2

Example 2: Find the average number of hours taken for all majors. Display the name of the

major, the number of students, and the average.

SELECT MAJOR, COUNT(*), AVG(CREDITS)

FROM STUDENT

GROUP BY MAJOR;

MAJOR COUNT(*) AVG(CREDIT)

Art 1 63

CIS 1 15

History 1 90

Math 3 29

Example 3: Find all courses in which fewer than three students are enrolled.

SELECT COURSENUM

FROM ENROLLGROUP BY COURSENUM

HAVING COUNT(*) < 3;

COURSENUM

CIS201A

HST205A

MTH101B

MTH103C

MULTIPLE TABLE QUERIES

A JOIN operation is performed when more than one table is specified in the FROM clause. You

would join two tables if you need information from both.

You must specify the JOIN condition explicitly in SQL. This includes naming the columns in

common and the comparison operator.

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Example 1: Find the name and courses that each faculty member teaches.

SELECT FACULTY.FACNAME, COURSENUM

FROM FACULTY, CLASS

WHERE FACULTY.FACID = CLASS.FACID;

FACULTY.FACNAME COURSENUM

Adams ART103A

Tanaka CIS201A

Byrne MTH101B

Smith HST205A

Byrne MTH103C

Tanaka CIS203A

When both tables have an attribute name in common, you must specify which version of the

attribute that you are referring to by preceding the attribute name with the table name and a

period. (e.g., table-name.col-name). This is called "qualification".

It is sometimes more convenient to use an "alias" (an alternative name) for each table. SQL

specifies alias names in the FROM clause immediately following the actual table. Once defined,

you can use the alias anywhere in the SELECT where you would normally use the table name.

Example 2: Find the course number and the major of all students taught by the faculty member

with ID number 'F110'. (3 table JOIN)

SELECT ENROLL.COURSENUM, LNAME, MAJOR

FROM CLASS , ENROLL, STUDENT

WHERE FACID = 'F110'

AND CLASS.COURSENUM = ENROLL.COURSENUM

AND ENROLL.STUID = STUDENT.STUID;

ENROLL.COURSENUM LNAME MAJOR

MTH101B Rivera CIS

MTH103C Burns ART

MTH103C Chin Math

Using aliases, this would be:

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SELECT E.COURSENUM, LNAME, MAJOR

FROM CLASS C, ENROLL E, STUDENT S

WHERE FACID = 'F110'

AND C.COURSENUM = E.COURSENUM

AND E.STUID = S.STUID;

NESTED QUERIES

SQL allows the nesting of one query inside another, but only in the WHERE and the HAVING

clauses. In addition, SQL permits a subquery only on the right hand side of an operator.

Example 1: Find the names and IDs of all faculty members who teach a class in room 'H221'.

You could do this with 2 queries as follows:

SELECT FACIDFROM CLASS

WHERE ROOM = 'H221'; ---> RESULT: F101, F102

SELECT FACNAME, FACID

FROM FACULTY

WHERE FACID IN (F101, F102);

or you could combine the 2 into a nested query:

SELECT FACNAME, FACID

FROM FACULTYWHERE FACID IN

(SELECT FACID

FROM CLASS

WHERE ROOM = 'H221');

Note that the nested SELECT is executed first and its results are used as the argument to the

outer SELECTs IN clause.

FACNAME FACID

Adams F101

Smith F202

Example 2: Retrieve an alphabetical list of last names and IDs of all students in any class taught

by faculty number 'F110'.

SELECT LNAME, STUID

FROM STUDENT

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WHERE STUID IN

(SELECT STUID

FROM ENROLL

WHERE COURSENUM IN

(SELECT COURSENUM

FROM CLASSWHERE FACID = 'F110'))

ORDER BY LNAME;

LNAME STUID

Burns S1010

Chin S1002

Rivera S1020

The most deeply nested SELECT is done first. Thus, after the first select you have:

SELECT LNAME, STUID

FROM STUDENT

WHERE STUID IN

(SELECT STUID

FROM ENROLL

WHERE COURSENUM IN

('MTH101B','MTH103C'))

ORDER BY LNAME;

Next, the next most deeply is done.

SELECT LNAME, STUID

FROM STUDENT

WHERE STUID IN

('S1020','S1010','S1002')

ORDER BY LNAME;

Finally, the outer Select is executed giving the result printed above.

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Example 3: Find the name and IDs of students who have less than the average number of credits.

SELECT LNAME, STUID

FROM STUDENT

WHERE CREDITS