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Chapter 7 in Lab Reference

Introduction to PL/SQL

PL/SQL is the procedural extension to SQL.

PL/SQL is a programming language like C, Java or Pascal, used to access the database from various environments.

e.g. Forms, Reports to create triggers, procedures, functions, etc…

SQL-DML can be natively embedded in PL/SQL programs.

PL/SQL 2

Introduction to PL/SQL

PL/SQL provides high-level language features such as block structure, conditional statements, loop statements, variable types, structured data and customized error handling.

PL/SQL is integrated with the database server. It does not exist as a standalone language.

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Basic Structure of PL/SQL

The basic unit in PL/SQL is a block.

All PL/SQL programs are made up of blocks, which can be nested within each other.

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Basic Structure of PL/SQL

DECLARE    

/* Declarative section: variables, types, and local subprograms. */    

BEGIN   

/* Executable section: procedural and SQL-DML statements go here. */

/* This section of the block is required. */    

EXCEPTION    

/* Exception handling section: error handling statements go here. */    

END;

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Basic Structure of PL/SQL

Only the executable section is required. The other sections are optional.

The only SQL statements allowed in a PL/SQL program are SELECT, INSERT, UPDATE, DELETE and several other data manipulation statements plus some transaction control.

Data definition statements like CREATE, ALTER, or DROP are not allowed.

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Basic Structure of PL/SQL

The executable section also contains constructs such as assignments, branches, loops, procedure calls, and triggers.

PL/SQL is not case sensitive.

C style comments (/* ... */) may be used, and for a one line comment precede it with (--).

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Variables and Types

Types in PL/SQL are the same as in SQL.

Variables can be declared in the following ways:

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Declare id NUMBER;

name VARCHAR(20);

1.

Variables and Types

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Declare id emp.empno%TYPE;

name emp.ename%TYPE; /* TableName.ColumnName%TYPE */

2.

Declare depttuple dept%ROWTYPE;

/* depttuple will be a record that contains fields that represent the

columns in table dept. */

/* depttuple(deptno,dname,location) */

3.

Variables and Types

A variable can be initialized:

A variable can be constrained to not null:

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Declare a NUMBER :=3;

Declarea NUMBER NOT NULL :=3;

Declare a NUMBER NOT NULL DEFAULT 3;

or

Select Statement

Select statement has a different form:

Select should return a single tuple, if several tuples are required, use a Cursor.

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SELECT column INTO variablesFROM tableWHERE condition ;

Simple Program

Using the following table declaration:

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CREATE TABLE T1(     e NUMBER,     f NUMBER );

INSERT INTO T1 VALUES ( 1, 3 ); INSERT INTO T1 VALUES ( 2, 4 );

e f

1 3

2 4

T1

Simple Program

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DECLARE    a NUMBER;    b NUMBER;

BEGIN     SELECT e, f INTO a, b FROM T1 WHERE e>1;     INSERT INTO T1 VALUES ( b, a );

END;

e f

1 3

2 4

T1 before

e f

1 3

2 4

4 2

T1 after

Control Flow in PL/SQL

IF – THEN – END IF

LOOP – EXIT WHEN – END LOOP

FOR – END LOOP

WHILE – END LOOP

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IF – THEN – END IF

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IF condition_1 THEN actions_1; [ ELSE actions_last ; ]END IF;

IF condition_1 THEN actions_1; [ ELSIF condition_2 THEN actions_2 ; ] [ ELSIF condition_n THEN actions_n ; ] [ ELSE actions_last ; ]END IF;

Syntax

1.

2.

IF – THEN – END IF

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DECLARE     a NUMBER;     b NUMBER;

BEGIN    

SELECT e, f INTO a, b FROM T1 WHERE e>1;    

IF b=1 THEN  INSERT INTO T1 VALUES ( b, a );     ELSE  INSERT INTO T1 VALUES ( b+10, a+10 );     END IF;

END;

Example

e f

1 3

2 4

T1 before

e f

1 3

2 4

14 12

T1 after

LOOP – EXIT WHEN – END LOOP

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LOOP     /* list of statements */ EXIT WHEN condition ; /* list of statements */END LOOP;

Syntax

LOOP – EXIT WHEN – END LOOP

Insert each of the pairs (1, 1) through (100, 100) into T1.

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Example

DECLARE     i NUMBER := 1;

BEGIN    

LOOP         INSERT INTO T1 VALUES ( i, i );         i := i+1;         EXIT WHEN i>100;     END LOOP;

END;

FOR – END LOOP

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FOR counter_variable IN [REVERSE] lower_bound .. upper_bound LOOP         /* list of statements for FOR loop body */ END LOOP;

-- counter_variable will be declared implicitly.

Syntax

WHILE – END LOOP

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WHILE condition LOOP         /* list of statements for WHILE loop body */ END LOOP;

Syntax

Cursors

Cursor stands for “Current set of records”.

A cursor is a variable that runs through the tuples of some relation.

By fetching into the cursor each tuple of the relation, we can write a program to read and process the value of each such tuple.

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Cursor Example

For example, delete every tuple whose first component is less than the second, and insert the reverse tuple into T1.

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DECLARE    

/* Output variables to hold the result of the query */    a T1.e%TYPE;     b T1.f%TYPE;         /* Cursor declaration */  CURSOR T1Cursor IS           SELECT e, f FROM T1 WHERE e < f         FOR UPDATE;

e f

1 3

2 4

T1

Cursor Example

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BEGIN    

OPEN T1Cursor;

LOOP       /* Retrieve each row of the result of the above query into PL/SQL variables */ FETCH T1Cursor INTO a, b;           

/* If there are no more rows to fetch, exit the loop */EXIT WHEN T1Cursor%NOTFOUND;        

/* Delete the current tuple */ DELETE FROM T1 WHERE CURRENT OF T1Cursor;            

/* Insert the reverse tuple */ INSERT INTO T1 VALUES ( b, a );

END LOOP;        

CLOSE T1Cursor; /* Free cursor used by the query */

END;

e f

1 3

2 4

T1 before

e f

3 1

4 2

T1 after

Procedures Syntax

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CREATE [OR REPLACE] PROCEDURE procedure_name (parameters) IS /* No declare keyword */ local_var_declarations

BEGIN     /* list of statements for procedure body */ END procedure_name ;

Parameters: variable_name mode data_type

Modes: IN, OUT or INOUT.

Types should be unconstrained, i.e. char and varchar should be used instead of char(10) and varchar(20).

Procedure Example

Example:

This creates the procedure, to execute it:

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CREATE PROCEDURE addtuple1( i IN NUMBER, j IN NUMBER ) IS BEGIN     INSERT INTO T1 VALUES ( i, j ); END addtuple1;

BEGIN addtuple1( 99,100 ); END;

Procedure Example

Example:

This creates the procedure, to execute it:

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CREATE PROCEDURE addtuple3( a IN NUMBER, b OUT NUMBER ) IS BEGIN     b := 4;     INSERT INTO T1 VALUES ( a, b ); END addtuple3;

DECLARE     v NUMBER; BEGIN     addtuple3 (10, v ); END;

Functions Syntax

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CREATE [OR REPLACE] FUNCTION function_name ( parameters ) RETURN return_type IS /* No declare keyword */ Local_var_declarations

BEGIN /* list of statements for procedure body */ Return return_variable; END function_name ;

Procedures & Functions

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To find out what procedures and functions you have created, use the following SQL query:

Select object_type, object_name From user_objects Where object_type = 'PROCEDURE'  or object_type = 'FUNCTION';

To drop a stored procedure/function:

Drop procedure procedure_name ;

Drop function function_name ;

Error Handling

Two types of exceptions:

Predefined exceptions.

User-Defined exceptions.

Predefined exceptions: INVALID_CURSOR, NO_DATA_FOUND, INVALID_NUMBER,

ZERO_DIVIDE,TOO_MANY_ROWS, ROWTYPE_MISMATCH, etc.

Exceptions must be:

Declared in the declarative section of the block, Raised in the executable section, and Handled in the exception section.

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User Defined Exceptions

User-defined exceptions are declared in the declarative section.

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DECLAREexception_name EXCEPTION; -- exceptions are declared here

BEGIN executable_statements -- exceptions are raised here

[ EXCEPTION exception_handling ] -- exceptions are handled here

END;

User Defined Exceptions

PL/SQL 31

DECLAREe_toomany EXCEPTION; -- exception are declared here

BEGINIF num_students > 100 THEN RAISE e_toomany; -- exception is raised here

-- Any code here is not executed, if the condition is true

EXCEPTION

WHEN e_toomany THEN -- Control passes to the exception handler ... -- Code here will be executed

END;

Exceptions

When an exception is raised, control immediately passes to the exception section of the block.

Once control passes to the exception handler, there is no way to return to the executable section of the block.

Predefined exceptions are automatically raised when the associated error occurs.

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Handling Exceptions

PL/SQL 33

….

EXCEPTION

WHEN exception_name1 THEN Statements1; -- do something here

WHEN exception_name2 THEN Statements2;

WHEN OTHERS THEN Statements3;

END;

Handling Exceptions

A single handler can also be executed for more than one exception:

 

 

PL/SQL 34

….

EXCEPTION

WHEN NO_DATA_FOUND OR TOO_MANY_ROWS THEN

INSERT INTO log_table (info) VALUES ( 'A select error occurred.‘ );

END;

Triggers

A trigger defines an action the database should take when some database-related event occurs (events such as inserts, updates, deletes).

Triggers are similar to procedures, in that they are named PL/SQL blocks.

Differences between Procedures and Triggers:

A procedure is executed explicitly from another block via a procedure call with passing arguments,

while a trigger is executed (or fired) implicitly whenever the triggering event happens (DML: INSERT, UPDATE, or DELETE), and a trigger doesn't accept arguments.

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Basic Trigger Syntax

PL/SQL 36

CREATE [OR REPLACE] TRIGGER trigger_name     { BEFORE | AFTER | INSTEAD OF } { INSERT [OR] | DELETE [OR] | UPDATE } [ OF column_name ] ON table_name   

[ REFERENCING [ NEW AS new_row_name ] [ OLD AS old_row_name ] ]    

[ FOR EACH ROW [ WHEN ( trigger_condition ) ] ]    BEGIN /* list of statements for trigger body */ END trigger_name ;

Basic Trigger Syntax

You may specify up to three triggering events using the keyword OR.

Furthermore, UPDATE can be optionally followed by the keyword OF and a list of attribute(s) in <table_name>. If present, the OF clause defines the event to be only an update of the attribute(s) listed after OF. For example:    

If FOR EACH ROW option is specified, the trigger is row-level; otherwise, the trigger is statement-level.

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... INSERT ON R ...    

... INSERT OR DELETE OR UPDATE ON R

... UPDATE OF A, B OR INSERT ON R ...

Basic Trigger Syntax

Only for row-level triggers:

The special variables NEW and OLD are available to refer to new and old tuples respectively. Note: In the trigger body, NEW and OLD must be preceded by a colon (":"), but in the WHEN clause, they do not have a preceding colon!

The REFERENCING clause can be used to assign aliases to the variables NEW and OLD.

A trigger restriction can be specified in the WHEN clause, enclosed by parentheses. The trigger restriction is a SQL condition that must be satisfied in order for Oracle to fire the trigger. This condition cannot contain subqueries. Without the WHEN clause, the trigger is fired for each row.

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Trigger Example

We create a trigger that may insert a tuple into T5 when a tuple is inserted into T4. Specifically, the trigger checks whether the new tuple has a first component 10 or less, and if so inserts the reverse tuple into T5:

PL/SQL 39

CREATE TABLE T4 ( a INTEGER, b CHAR(10) );

CREATE TABLE T5 ( c CHAR(10), d INTEGER );

CREATE TRIGGER trig1    AFTER INSERT ON T4    

REFERENCING NEW AS newRow    

FOR EACH ROW     WHEN ( newRow.a <= 10 )    

BEGIN         INSERT INTO T5 VALUES ( :newRow.b, :newRow.a );     END trig1;