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SQLJ Training


SQLJ

SQLJ is a set of programming extensions that allow a programmer using the Java™ programming language to embed statements that provide SQL (Structured Query Language) database requests.

The two standard methods for accessing relational data from a Java program are SQLJ and Java Database Connectivity (JDBC).


SQLJ versus JDBCSQLJ programs require fewer lines of code than JDBC programs.

They are shorter, and hence easier to debug. SQLJ can perform syntactic and semantic checking on the code, using database connections at compile time.

SQLJ provides strong type-checking of query results and other return parameters, while JDBC values are passed to and from SQL without having been checked at compile time.


Where Can You Use SQLJ?

Applications

Applets

Servlets

JavaServer Pages

Enterprise JavaBeans


SQLJ ComponentsSQLJ consists of two components: the translator and the runtime libraries.

The translator reads a Java source file containing embedded SQL statements, which are then translated into calls to the SQLJ

runtime libraries.

These calls perform the actual database operations. SQLJ is an ISO and ANSI standard that was developed by many large corporations including Oracle, Sun, Microsystems, IBM, Compaq, Informix, and Sybase.


Requirements for Using SQLJ

SQLJ translator

JDBC drivers

A version of the Sun Microsystems Java Development Kit (JDK)

Database


Steps when using SQLJ1. Import the Java packages that contain SQLJ and JDBC

methods

2. Declare variables for sending data to or retrieving data from DB2 tables

3. Connect to a data source

4. Execute SQL statements

5. Handle SQL errors and warnings

6. Disconnect from the data source


SQLJ Programs

A SQLJ program, like any other Java program, is divided up into blocks, and a SQLJ statement may appear anywhere that a normal Java statement may appear.

All SQLJ statements begin with the language token #sql to differentiate those statements from other Java program statements.


Basics of SQLJ

•To use SQLJ in a Java program to access DB2, you need to take a few steps before you begin coding.

• Include the following files either in a directory with your applications or in your CLASSPATH.

• db2jcc.jar, which provides a JDBC driver (Type 4)

• sqlj.zip, which provides the SQLJ translator class files

• Location: SQLLIB/java


import statements

import java.sql.*;

import sqlj.runtime.*;

import sqlj.runtime.ref.*;


Basic SQLJ syntax

Any embedded SQLJ statement must conform to two simple rules:

1. The statement must be preceded by the syntax #sql.

2. The statement must end with a semi-colon ( ; ).


You should also enclose the SQLJ statements within braces, and though it's optional, include the context in which the statement should be executed.

#sql [context] {DELETE FROM EMP_ACT};


Passing information from an application

To insert into the EMP_ACT table, use the following syntax:

void m (String empno, String projno, int actno) throws SQLException

{

#sql [context] {INSERT INTO EMP_ACT (EMPNO, PROJNO, ACTNO)

values (:empno, :projno, :actno)};

}


Compiling SQLJ

• Command:

sqlj SqlJDemo.sqlj

• The sqlj translator creates a file named:

SqlJDemo.java


Statements that return a value

int result; #sql result = { VALUES update_product_price_func(1, 2) };

If an embedded SQL statement does return a result, you need a way to specify where that result should be placed. SQLJ syntax accommodates this need.


Host Variables and ExpressionsHost variables allow SQLJ programs to exchange information between the embedded SQL statements and the rest of the Java program.

A host variable is any Java variable that is declared in the Java program.

Host variables may be referenced within a SQLJ statement, and SQLJ takes care of the details of moving data back and forth between the SQL and Java environments .

:[mode] host_variable


Host Variables:[mode] host_variable

mode (optional) Specifies the mode of the host expression and may be set to one of the following:

IN The SQLJ statement may only read the value stored in the host variable (the value may not be changed).

OUT The SQLJ statement should write a new value to the host variable.

INOUT The SQLJ statement may both read and write the value of the host variable host_variable

The name of a Java variable in the program


Single-Row Queries// declare host variables

int id = 2;String first_name = null;String last_name = null;java.sql.Date dob = null;String phone = null;

// perform SELECT to get the customer details for the customer #2// from the customers table

#sql {SELECTfirst_name, last_name, dob, phoneINTO:first_name, :last_name, :dob, :phoneFROMcustomersWHEREid = :id};


Updating Rows

The SQL UPDATE statement is used to modify rows in a table.

When an UPDATE statement is used in a SQLJ executable statement, host expressions can appear in the SET and WHERE clauses


Updating Rowsint new_quantity = 10;int cust_id = 2;int prod_id = 3;#sql {

UPDATEpurchasesSETquantity = :new_quantityWHEREpurchased_by = :cust_idANDproduct_id = :prod_id

};


Deleting Rows

The SQL DELETE statement is used to remove rows from a table. When a DELETE statement is used in a SQLJ executable statement, host expressions can appear in the WHERE clause.

#sql {

DELETE FROM

customers

WHERE

id = :cust_id

};


Inserting Rows

The SQL INSERT statement is used to add rows to a table.

When an INSERT statement is used in a SQLJ executable statement, host expressions can appear in the VALUES clause.


Inserting Rows

int id = 13;int type_id = 1;String name = "Life Story";String description = "The Life and Times ";double price = 199.95;#sql {INSERT INTOproducts (id, type_id, name, description, price)VALUES(:id, :type_id, :name, :description, :price)};


Inserting Rowsint id = 14;double new_price = 10.95;int prod_id = 4;#sql {INSERT INTO productsSELECT:id, type_id, name, description, :new_priceFROMproductsWHEREid = :prod_id};


Handling Database Null ValuesUnfortunately, the Java numeric, logical, and bit types (int, float, boolean, and byte, for example) can't retrieve nulls from the database.

So what do you do if a column you want to select using a SQLJ statement may contain a null?

You must use the Java wrapper classes. A wrapper class is a Java class that allows you to define a wrapper variable, which can then be used to retrieve database nulls.


Handling Database Null Values// set the price to null#sql { UPDATE productsSETprice = NULLWHEREid = 1};// retrieve the null price into price_var#sql { SELECT priceINTO :price_varFROM productsWHEREid = 1};


Handling Exceptions

SQLJ executable statements must be contained in a try/catch statement.

The try block contains the SQLJ statements that may cause a java.sql.SQLException, and the catch block should contain the statements to be executed when a java.sql.SQLException is raised.


SQLException

try {

#sql {

DELETE FROM

customers

};

} catch (SQLException exception) {

System.out.println("SQLException " + exception);

}


java.sql.SQLNullException

Thrown when a database null value is selected into a Java primitive type.

The try/catch statement in this example contains handlers for SQLNullException and SQLException.

Notice that SQLNullException is placed before SQLException


Transactions#sql { COMMIT [WORK] };#sql { ROLLBACK [WORK] }; The syntax elements are as follows: COMMIT

Commits a transaction, making the changes permanent. ROLLBACK

Rolls back a transaction, returning the database to the state it was in when the transaction first began. The effects of all SQL statements issued during the transaction will be erased.

WORK An optional word that is part of the supported SQL syntax.


Transactions

#sql {

INSERT INTO

customers (id, first_name, last_name, dob,

phone)

VALUES

('7', ‘Girish', ‘Verma', '03-FEB-1978',

'123456')

};

#sql { ROLLBACK };


Queries That Return Multiple RowsThe SELECT statement is used to retrieve rows from a database source such as a table or a view.

There are two cases to consider: a SELECT that returns one row, and a SELECT that returns multiple rows.

For the single row case, you can use the SELECT INTO statement described earlier in this chapter. For the case involving multiple rows, you must use a SQLJ iterator.


SQLJ IteratorsAn iterator is used to process multiple rows retrieved by a database query. From a conceptual point of view, a SQLJ iterator is similar to a PL/SQL cursor.There are five steps that you must perform to use an iterator to process rows returned by a SELECT statement:

1. Declare the iterator class.

2. Declare an iterator object from the iterator class.

3. Populate the iterator object using a SELECT statement.

4. Read the rows from the iterator object.

5. Close the iterator object.


SQLJ Iterators

There are two types of iterator classes: named

Both the Java variable type and the iterator column name used to store each column retrieved from the database must be specified in the iterator class.

positional Only the types of the Java variables used to store the

columns retrieved from the database are specified in the iterator class.

You do not specify column names


Named IteratorsWhen declaring a named iterator class, both the Java variable type and the host variable name used to store each column retrieved from the database must be specified.

The syntax for declaring a named iterator class is as follows:

#sql [modifiers] iterator class_name [implements_clause] [with_clause] (java_type column_name [, java_type column_name ...]);


Named Iteratorsmodifiers

The Java class modifiers: public, private, protected, and static. class_name

Specifies the name that you want to give the iterator class. implements_clause

Specifies the interfaces that the iterator class implements. This corresponds to the Java implements clause and is discussed later in this chapter.

with_clause Specifies a list of constants to define and initialize. The with

clause is discussed later in this chapter. java_type

Specifies the Java type of the iterator column represented by column_name.

column_name Specifies the name of the iterator column


Declare the iterator class

#sql private static iterator

ProductIteratorNamedClass

( int id,

double price,

String name,

String description );


Declare an iterator object from theiterator class

Java statement declares an iterator object named product_iterator from ProductIteratorNamedClass:

ProductIteratorNamedClass product_iterator;

The product_iterator object is now ready to be populated with the results of a SQL SELECT statement.


Populate the iterator object using aSELECT statement

#sql product_iterator = {

SELECT

id, name, description, price

FROM

products

WHERE

id <= 5

};


Read the rows from the iteratorobject

while (product_iterator.next( )){// print the contents of the iterator rowSystem.out.println("id = " + product_iterator.id( )); System.out.println("name = " + product_iterator.name( ));System.out.println("description = " + product_iterator.description( ));System.out.println("price = " + product_iterator.price( )); } // end of while loop


Close the iterator object

Once processing of an iterator is complete, the iterator must be closed using the close( ) method.

product_iterator.close( );


Scrollable Iterators

A scrollable iterator gives you the freedom to access the rows stored in the iterator in non-sequential order.

In order for an iterator class to support scrollable functionality, it must implement the sqlj.runtime.Scrollable interface.


Scrollable Iterators


NamedScrollableIterClass

implements sqlj.runtime.Scrollable (

int id,

String name

);

NamedScrollableIterClass

my_scrollable_named_iter;


Scrollable positional iterators


PosScrollableIterClass

implements sqlj.runtime.Scrollable (

int,

String

);

PosScrollableIterClass

my_scrollable_pos_iter;


Scrollable positional iteratorsFETCH PREVIOUS or FETCH PRIOR

Accesses the previous row. FETCH FIRST

Accesses the first row. FETCH LAST

Accesses the last row. FETCH ABSOLUTE :(row_number)

Accesses the row specified by row_number. The parameter row_number can be any expression that evaluates to a Java int value.

FETCH RELATIVE :(relative_row_number) Positions the iterator a specified number of rows forward or

backward from the current row.


SQL Procedures

You can invoke the procedure from a SQLJ program by using the CALL statement. #sql { CALL procedure_name([parameter_list])}; The syntax elements are as follows: procedure_name

Specifies the name of the PL/SQL stored procedure

parameter_listSpecifies a comma-delimited list of parameters to pass to the

PL/SQL stored procedure

#sql { CALL update_product_price(1, .9) };


SQL FunctionsFunctions must return a value, you should use a Java variable to store the result. #sql host_variable = { VALUES (

function_name([ parameter_list])) };

The syntax elements are as follows:host_variable

Specifies the host variable to use in storing the value returned

function_name Specifies the name of the function

parameter_list Specifies a list of parameters to be passed to the function


SQL Functions

int result;

#sql result = {

VALUES(update_product_price_func(1,

2)) };

Or

#sql result = { VALUES

update_product_price_func(1, 2) };


The SET StatementThe SET statement is used to assign a value to a host expression. The syntax for a SQLJ executable statement that uses the SET statement is: #sql { SET :host_expression = expression }; The syntax elements are as follows:host_expression

Specifies the host expression. This has a default mode of OUT and may not be set to IN or INOUT (doing so will result in a translation time error).

expressionSpecifies any valid SQL statement that returns a value that

can be assigned to the host expression,


The SET Statement

int result_sum = 0;

#sql {

SET :result_sum =

update_product_price_func(1, .9) +

update_product_price_func(2, .8)

};


The SET Statement

Date system_date = null; #sqlj { SET :system_date = sysdate };


Database ObjectsA class is known as an object type, and it's from an object type that actual database objects are instantiated.

In order to access database objects using SQLJ, you must create special custom classes.

These custom classes mirror the database types on which the database objects are based, and are used to declare host objects in your SQLJ program.

A host object is similar to a host variable. It resides locally in your Java program and may be used to access database objects.


Defining Object Types

CREATE TYPE t_address AS OBJECT (

street VARCHAR2(15), city

VARCHAR2(15), state CHAR(2), zip

VARCHAR2(9) );


Defining Object Types

CREATE TYPE t_customer AS OBJECT (

id NUMBER,

first_name VARCHAR2(10),

last_name VARCHAR2(10),

dob DATE,

phone VARCHAR2(15),

address t_address,

-- the function get_age( ) returns the age of the customer

in years

MEMBER FUNCTION get_age RETURN INTEGER

);


Defining Object TypesCREATE TYPE BODY t_customer AS

-- the function get_age( ) returns the age of the customer in yearsMEMBER FUNCTION get_age RETURN INTEGER IS

age INTEGER;BEGIN

-- calculate the age in years

SELECT ROUND(((sysdate - dob) / 365), 0) INTO age FROM dual;

RETURN age;

END;

END;


Object Columns and Object Tables

You can use object types to define columns in a table.

Such columns are known as object columns because they contain objects rather than scalar values.

You can also define tables so that each row in the table represents an instance of an object. Such tables are known as object tables.


Object Columns

An object column is a column in a table that is defined using an object type

CREATE TABLE customers ( customert_customer );


Object Tables

An object table is a table in which each row contains one object of a given object type. Such objects are known as row objects.

You create an object table using the CREATE TABLE OF statement.


Adding Objects to TablesWhen adding a new row object or column object to a table, you must place the values for the object in a constructor for the object type.

INSERT INTO

customers (customer)

VALUES (

t_customer(1, 'Y', 'P', '03-FEB-1978', '1234455',

t_address('1 ABC', 'EFG', 'HI', '12345')

)

);


Retrieving Objects

When referencing an individual attribute in an object column, you must use an alias for the table in your SELECT statement.

SELECT c.customer

FROM customers c

WHERE c.customer.id = 1;


Invoking Object Methods

SELECT c.customer.get_age( )

FROM customers c

WHERE c.customer.id = 1;

SELECT c.get_age( )

FROM customers2 c

WHERE c.id = 1;


Modifying Objects

You can modify an attribute of an object using an UPDATE statement.

UPDATE

customers c

SET

c.customer.first_name = 'Yashodip'

WHERE

c.customer.id = 1;


Accessing Database Objects UsingSQLJ

Adding Objects

There are two types of database objects: column objects and row objects


Adding a column objectYou can add a column object to a table using one of two methods:

Use the object type constructor to set the object attributes in an INSERT statement.

Use a host object in an INSERT statement. This method involves three steps:

Create a host object using the appropriate custom class.Set the host object attributes using the set mutator methods.Use the host object in an INSERT statement.


Adding a column object

#sql {

INSERT INTO

customers

VALUES (

t_customer(1, 'Y', 'P', '03-FEB-1978', '1234455',

t_address('1 ABC', 'EFG', 'HI', '12345')

)

};


Adding a row object

Adding a row object may be performed by setting the object attributes directly in an INSERT statement, just as you would set the columns for a non-object table.


Retrieving Objects

Retrieval of a column or row object from a table may be accomplished using the following steps:

Declare a host object using the custom class for the object type that you wish to retrieve.

Using a SELECT INTO statement or an iterator, retrieve the column or row object into the host object.

Read the host object's attributes using the get accessor methods.


Declaring a host objectFirst, you need to declare a host object to store the column or row object to be retrieved.

TCustomer customer;

Once you have your host object, you can use a SELECT INTO statement to retrieve a columnor row object into that host object.

The syntax of that SELECT INTO statement depends on whether you are retrieving a column object or a row object.


Retrieving a column object

#sql {SELECT

c.customerINTO

:customerFROM

customers cWHERE

c.customer.id = 1};


Retrieving a row object

#sql {SELECT

id, first_name, last_name, dob, addressINTO

:id, :first_name, :last_name, :dob, :addressFROMcustomers2WHEREid = 1};


Referencing object attributesOnce you've retrieved an object, you can access its attributes using the get accessor methods.

System.out.println("id = " + customer.getId( ));

System.out.println("first_name = " + customer.getFirstName( ));

System.out.println("last_name = " + customer.getLastName( ));


Modifying Object Attributes

You can modify the attributes of a column object or a row object using one of two possible methods:

Modify the attributes directly in an UPDATE statement.

Modify the attributes through a host object, and write this host object back into the database with an UPDATE statement.


Modifying attributes directly usingan UPDATE statement

#sql {

UPDATE

customers c

SET

c.customer.first_name = :first_name

WHERE

c.customer.id = 1

};


Modifying attributes through a hostobject

Declare a host object using the custom class.

Retrieve the original database object into the host object.

Modify the attributes in the host object using the set mutator methods.

Use the host object in an UPDATE statement to modify the original database object.


CollectionsThere are two types of collections.

The first is known as a VARRAY, which enables a variable-length array to be stored within a table.

The second is a nested table, which enables a table to be stored within another table.

These collection types allow rows that make up the detail part of a relationship to be grouped with the master rows.


VARRAYs

A VARRAY is an ordered set of elements. The maximum number of elements in a VARRAY is set when the VARRAY is created.

You create a VARRAY by first declaring a type, and then using that type to define a column in a table.


VARRAYs

CREATE TYPE t_address2 AS VARRAY(3)

OF VARCHAR2(50); /

CREATE TABLE customers3 (

id NUMBER,

first_name VARCHAR2(10),

last_name VARCHAR2(10),

addresses t_address2

);


INSERT INTO customers3 VALUES (1, ‘G', ‘V',

t_address2('1 a, b, c, 12345',

‘602 Magarpatta city,as, pune, 54321')

);


Retrieving the Contents of a VARRAY

SELECT *

FROM customers3

WHERE id = 1;


Referenceshttp://www.stanford.edu/dept/itss/docs/oracle/9i/java.920/a96655/toc.htm

http://www.javaworld.com/javaworld/jw-05-1999/jw-05-sqlj_p.html

http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0412cline/


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