Lecture 4

Introduction to PL/SQL Procedures & Cursors

Overview

Overview of PL/SQL Development of a coded block Interacting with an Oracle Database Controlling PL/SQL process flow Cursor handling

Embedding SQL

SQL is not functionally complete Lacks the full facilities of a programming language

variables, flow of control etc.

All DBMSs top up functionality by embedding SQL in a procedural language

But details vary from one DBMS to another. However, procedures and functions can be ported

between systems.

PL/SQL - Introduction

An Oracle-specific procedural extension to SQL, allowing for modularity, variable declaration, loops and logical constructs.

Allows for advanced error handling. Communicates natively with other Oracle

database objects. Managed centrally within the Oracle

database.

Why use PL/SQL?

Manage business rules – through middle layer application logic.

Generate code for triggers. Generate code for the user interface. Enable database-centric client/server

applications.

Centralised vs. Decentralised

Common copy of executed code – one copy to maintain

Multiple copies of executable code on the decentralised system – multiple copies to maintain leading to increase difficulty in maintaining the system

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End;Server

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Advantages of using PL/SQL to access Oracle PL/SQL is managed centrally within the database. Code is managed by the DBA, and execution

privileges are managed in the same way as with other objects.

PL/SQL objects are first-class Oracle DB objects. Easy to read:

with modularity features and error handling.

Centralised Control

Enables the DBA to: specify rules in one place (as a procedure,

function, trigger or package in PL/SQL); force user access through the predefined

PL/SQL, so users cannot write their own procedural code and use this instead e.g. define security privileges giving users access to

table(s) only through a particular procedure.

Using PL/SQL as a Programming Language

Permits all “flow of control” operations of standard programming languages, e.g. Jumps GOTO ConditionsIF-THEN-END IF;

IF-THEN-ELSE-END IF Loops LOOP-EXIT;

WHEN-END LOOP;FOR-END LOOP;WHILE-END LOOP

Allows extraction of data into variables and its subsequent manipulation.

Modules in PL/SQL

There are 4 types of modules in PL/SQL Procedure – a series of statements which may or

may not return a value. Function – a series of statements which must

return a single value. Trigger – a series of statements which is executed

after an event has triggered a condition. Package – a collection of procedures and

functions which has 2 parts: a listing and a body.

Use of Data Types Number – used to store any number. Char(size) & varchar2(size) e.g. char(10) –

used to store alphanumerical text strings; the char data type will pad the value stored to the full length declared.

Date – used to store dates and times. Long – used to store large blocks of text up

to 2 gigabytes in length (limited operations)

Non-DB Data Types

DEC, DECIMAL, REAL, INTEGER, INT – these are numerical data types that are a subset of number.

Binary_integer – binary format for number type but can not be stored in database unless converted first.

Character – same as char. Boolean – true/false value. Table/record – tables can be used to store the equivalent

of an array while records store the variables with composite data types.

SQL Scripts A set of commands to run in sequence. Stored as a text file (e.g. using Notepad) on disk

and not in the data dictionary. It is accessed by its file name using @ or Start.

Script called:

Create_lecturer_copy.sql

Executed by:

SQL> @U:\create_lecturer_copy

The SQL Procedure

A block of SQL statements stored in the Data Dictionary and called by applications.

Satisfies frequently-used or critical application logic. When called, all code within the procedure is executed

(unlike packages). Action takes place on the server, not the client. Does not (normally) return a value to the calling program. Not available in Oracle 6 or older. Aids security as DBA may grant access to procedures

rather than tables, therefore some users cannot access tables except through a procedure.

Building a Procedure: Contents

1. CREATE OR REPLACE command;

2. Object to be created;

3. Name of object;

4. Any variables accessed or imported;

5. Local variables declared;

6. Code block enclosed by BEGIN … END;

Create or replace procedure inflation_rise (inf_rate in number)

Begin

update employee

set salary = salary + (salary * inf_rate / 100);

commit;

End;

1. Create or replace command

2. Object to be created

3. Name of object

4. Any variables accessed or imported

5. Declared local variables

6. Code

This procedure is called inflation_rise and uses a variable accessed as inf_rate which is a number, this is passed in when the procedure is used. It simply updates the salary by the rate of inflation.

Compiling and Executing Procedures Like any program the code needs to be compiled. @inflation_rise

compiles the procedure from a file with this name; makes it available to the data base.

Execute inflation_rise executes the procedure.

Remember to re-compile a procedure after editing.

For ease of use, it is best to write procedures in Notepad, then they can be easily edited and you have a back-up copy.

ExampleCREATE OR REPLACE PROCEDURE validate_customer (v_cust VARCHAR) AS

v_count NUMBER;

BEGIN

SELECT COUNT(*) INTO v_count

FROM CUSTOMER

WHERE CUST_CODE = v_cust;

IF v_count > 0 THEN

DBMS_OUTPUT.PUT_LINE(‘customer valid’);

ELSE

DBMS_OUTPUT.PUT_LINE(‘customer not recognised’);

END IF;

END;

SQL Code Block

Variable passed into procedure

Local variable used by procedure

Cursors in SQL Enables users to loop round a selection of

data. Stores data select from a query in a temp

area for use when opened. Use complex actions which would not be

feasible in standard SQL selection queries

Declaring Cursors Declared as a variable in the same way as

standard variables. Identified as cursor type. SQL included, e.g.

Cursor cur_emp is

Select emp_id, surname ‘name’, grade, salary

From employee

Where regrade is true;

Cursors A cursor is a temporary store of data. The data is populated when the cursor is opened. Once opened, the data must be moved from the

temporary area to a local variable to be used by the program. These variables must be populated in the same order that the data is held in the cursor.

The data set is looped round till an exit clause is reached.

Current rowCurrent rowCursor

Active setActive set

7369 SMITH CLERK

7566 JONES MANAGER

7788 SCOTT ANALYST

7876 ADAMS CLERK

7902 FORD ANALYST

Cursor FunctionsCursor Functions

Controlling the CursorControlling the Cursor

• Create a Create a named named SQL areaSQL area

DECLAREDECLAREDECLAREDECLARE

• Identify Identify the active the active setset

OPENOPENOPENOPEN

• Load the Load the current current row into row into variablesvariables

FETCHFETCHFETCHFETCH

• Test for Test for existing existing rowsrows

EMPTY?

• Return to Return to FETCH if FETCH if rows rows foundfound

NoNo

• Release Release the active the active setset

CLOSECLOSECLOSECLOSEYesYes

Controlling the Cursor…Controlling the Cursor…Open the cursor.Open the cursor.

CursorCursor

PointerPointer

Fetch a row from the cursor.Fetch a row from the cursor.

CursorCursor

PointerPointer

Continue until empty.Continue until empty.

CursorCursor

PointerPointer

Close the cursor.Close the cursor.

CursorCursor

Cursor AttributesCursor Attributes

Attribute Type Description

%ISOPEN Boolean Evaluates to TRUE if the cursor is open

%NOTFOUND Boolean Evaluates to TRUE if the most recent fetch does not return a row

%FOUND Boolean Evaluates to TRUE if the mostrecent fetch returns a row; logicalcomplement of %NOTFOUND

%ROWCOUNT Number Evaluates to the total number of rows returned so far

To obtain status information about a cursor.

Create or replace procedure proc_test as

v_empid number;

Cursor cur_sample isSelect empid from employee where grade > 4;

Beginopen cur_sample;loopfetch cur_sample into v_empid;exit when cur_sample%notfound; update employee

set salary = salary + 500where empid = v_empid;

end loop;End;

Open cursor for use.

Loops round each value

returned by the cursor

Place the value from the cursor into the variable v_empid

Stop when no more records are found

25463

12245

55983

12524

98543

Data returned by cursor

Declare Cursor

Notepad file called:

Create_procedures.sql

1) Open SQL*Plus and logon

2) At the prompt enter:

@create_procedures

You will get a prompt which should say ‘procedure created’ otherwise use

SHOW ERRORS to view errors in the code.

3) To run the procedure enter:

Execute proc_test

4) If you check your data you should now find that the procedure has run successfully

Use of conditions IF statements can be used

If <condition> Then…..

End if;

E.g.

Remember to end the IF statement Use of indented code will make it easier to debug!

. . .IF v_ename = 'MILLER' THEN v_job := 'SALESMAN'; v_deptno := 35; v_new_comm := sal * 0.20; END IF;. . .

. . .IF v_ename = 'MILLER' THEN v_job := 'SALESMAN'; v_deptno := 35; v_new_comm := sal * 0.20; END IF;. . .

The %ISOPEN Attribute Can fetch rows only when the cursor is open. Use the %ISOPEN cursor attribute before

performing a fetch to test whether the cursor is open.

Example

IF NOT cur_sample%ISOPEN THENOPEN cur_sample;

END IF;LOOP FETCH cur_sample...

IF NOT cur_sample%ISOPEN THENOPEN cur_sample;

END IF;LOOP FETCH cur_sample...

DECLARE CURSOR emp_cursor IS SELECT empno, ename FROM emp; emp_record emp_cursor%ROWTYPE;BEGIN OPEN emp_cursor; LOOP FETCH emp_cursor INTO emp_record; ...

DECLARE CURSOR emp_cursor IS SELECT empno, ename FROM emp; emp_record emp_cursor%ROWTYPE;BEGIN OPEN emp_cursor; LOOP FETCH emp_cursor INTO emp_record; ...

Cursors and RecordsCursors and RecordsProcess the rows of the active set conveniently by fetching values into a PL/SQL RECORD.Example

Cursor FOR LoopsCursor FOR Loops

FOR record_name IN cursor_name LOOP

statement1;

statement2;

. . .

END LOOP;

FOR record_name IN cursor_name LOOP

statement1;

statement2;

. . .

END LOOP;

The cursor FOR loop is a shortcut to process cursors.Syntax

Implicitly opens, fetches, and closes cursor.The record is implicitly declared.

Cursor FOR Loops: An ExampleCursor FOR Loops: An Example

Retrieve employees one by one until no more areRetrieve employees one by one until no more areleft:left:

DECLARE CURSOR emp_cursor IS SELECT ename, deptno FROM emp;BEGIN FOR emp_record IN emp_cursor LOOP -- implicit open and implicit fetch occur IF emp_record.deptno = 30 THEN ... END LOOP; -- implicit close occursEND;

DECLARE CURSOR emp_cursor IS SELECT ename, deptno FROM emp;BEGIN FOR emp_record IN emp_cursor LOOP -- implicit open and implicit fetch occur IF emp_record.deptno = 30 THEN ... END LOOP; -- implicit close occursEND;