Database SystemsDatabase Systems

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Server side Database Server side Database Programming using PL/SQLProgramming using PL/SQL

Version 1.1Version 1.1

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What is PL/SQL? SQL

Structured language to access databaseANSI StandardDeclarative

○ Specifies what to access but not how to - SELECT id, first_name, family_name FROM

authors PL/SQL

Developed by Oracle as a procedural extension to SQL

Declare variables, IF..ELSE, WHILE & FOR loops, functions, procedures and other features found in a programming language

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Server Side Programming PL/SQL executes inside DBMS

DBMS maintains Relational dataAlso stores and executes Procedural code!

AdvantagesResults from one query can be used as a

basis for the next query without having to pull data from DBMS to client side for processing!

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Client Side Procedural logic DBMS maintains relational data Client side programs implement procedural

logic

Is there a problem? Performance impact due increased network

traffic Cumulative effect if many clients run at the same

time Code re-use may not be possible

Many applications may incorporate processing that are quite similar but sharing may not be easy!

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PL/SQL - Basics Block structured

Basic program unit is a blockContains variables, code and error handler

A BLOCK is contained within BEGIN and END statements with executable commands in betweenMust contain some commands, even if they do

nothing! PL/SQL programs must at least contain 1

block Blocks can be nested (block within another

block)

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PL/SQL – Block Syntax

DECLARE

variable declarations

BEGIN

program code

EXCEPTION

exception handler code

END;

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Optional

PL/SQL - EXCEPTIONs EXCEPTIONS

“unexpected errors” that occur during execution occurs at run-time not at compile time!

EXCEPTION HANDLER Code that executes when EXCEPTION occursMakes the code more robust

Oracle Server has many pre-defined errorsno_data_found, value_error, too_many_rows, others

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PL/SQL - Anonymous BlockDECLARE

today date;

BEGIN

SELECT sysdate INTO today FROM dual;

DBMS_OUTPUT.PUT_LINE (‘Today -’ || today);

END;

• What exactly happens in this code?• What is DUAL? (revision test !)

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PL/SQL - Named Block Has a name and stored in Oracle Server Contains Header section

name, key word - a function, procedure or triggertype of value it returns in case of function

At the time of creation, the code within the named block is NOT executed but compiled and stored in Oracle Server

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PL/SQL - Data type - CharacterDECLARE

family_name VARCHAR2 (20);

Age NUMBER(3);

Assignment

family_name := ‘Anderson’;

Age := 21;

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PL/SQL – %Type %TYPE

To map a variable directly to the same datatype as the table column

DECLAREauthor_id AUTHORS.ID%TYPE;

PL/SQL variable ‘author_id’ is of same datatypewhich is used to define column name ‘id’ of table AUTHORS.

If column type changes PL/SQL Code would still work! {Example: VARCHAR2(20) to VARCHAR(30)}

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PL/SQL - Scope Rules

Variables, procedures and functions can be referenced by the code executing inside the block in which they are defined

Understanding of scope of variables, functions is especially important in the context of nested blocks!

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PL/SQL Scope ExampleDECLARE

father_name VARCHAR2(20):='Patrick';date_of_birth DATE:='20-Apr-1972';

BEGINDECLARE child_name VARCHAR2(20):='Mike'; date_of_birth DATE:='12-Dec-2002';

BEGIN DBMS_OUTPUT.PUT_LINE('Father''s Name: '||

father_name); DBMS_OUTPUT.PUT_LINE('Date of Birth: '||date_of_birth); DBMS_OUTPUT.PUT_LINE('Child''s Name: '||child_name);END;

DBMS_OUTPUT.PUT_LINE('Date of Birth: '||date_of_birth);END;/

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PL/SQL Operators Expressions consist of PL/SQL operators

and operandsArithmetic Operators

○ **, *, /, +, -Comparison Operators

○ =, <>, !=, <, >, <=, >=, LIKE, BETWEEN, IN, IS NULL

Logical Operators○ AND, OR, NOT

String Operator – Concatenation using ‘||’

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PL/SQL – NULL

NULL means “UNKNOWN” value Use IS NULL or IS NOT NULL to check

for NULL value NULL value comparison with ‘= NULL’

or ‘!= NULL’ may produce unpredictable results!

Use NVL function when appropriateNVL (<expression>, <value if expression is NULL>)

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PL/SQL – SELECT sampleDECLARE name VARCHAR2(20); surname VARCHAR2(20);BEGIN SELECT first_name, family_name INTO name, surname FROM AUTHORS WHERE id = 1 ; DBMS_OUTPUT.PUT_LINE (‘Row selected is : ‘ || name || ‘-’ || surname);END;

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Performing DML Operations from PL/SQL (INSERT, UPDATE, and DELETE)

You can write INSERT, UPDATE, and DELETE statements directly in PL/SQL programs, without any special notation:

%ROWCOUNT Attribute: How Many Rows Affected So Far?

ExampleSET SERVEROUTPUT ON; BEGIN UPDATE employees SET salary = salary * 1.05

WHERE ...; dbms_output.put_line('Updated ' || SQL%ROWCOUNT || '

salaries.'); END; /

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PL/SQL – Program Flow Control Conditional execution

IF-THEN, IF-THEN-ELSE, IF-THEN-ELSIFCASE

Repeated execution until some condition LOOP-END LOOP, FOR-LOOP-END

LOOPWHILE-LOOP-END LOOPEXIT WHEN

Jump to code section GOTO

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PL/SQL – Conditional ExecutionIF <condition> THEN statement1; statement2; …..END IF; can be evaluated to TRUE, FALSE or

NULL – statement1, statement2 etc., are executed only if it evaluates to TRUE

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IF Example

IF sales > quota THEN compute_bonus(empid);

UPDATE payroll SET pay = pay + bonus WHERE empno = emp_id;

END IF;

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PL/SQL – IF-THEN-ELSE

IF <condition> THEN statement1; statement2; …..ELSE statement3; statement4; …..END IF;

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PL/SQL IF-THEN-ELSIF

IF <condition1> THEN statement1; …..ELSIF <condition2> statement3; ……ELSE …..END IF;

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Example

BEGIN ... IF sales > 50000 THEN bonus := 1500; ELSIF sales > 35000 THEN bonus :=

500; ELSE bonus := 100; END IF; INSERT INTO payroll VALUES

(emp_id, bonus, ...); END;

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PL/SQL – Simple Loop The EXIT-WHEN statement lets you complete a

loop if further processing is impossible or undesirable.

When the EXIT statement is encountered, the condition in the WHEN clause is evaluated.

If the condition is true, the loop completes and control passes to the next statement.E.G.

LOOP statement1; …… EXIT {WHEN ….} ;END LOOP;

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PL/SQL LOOP Example In the following example, the loop

completes when the value of total exceeds 25,000:

LOOP ... total := total + salary; EXIT WHEN total > 25000; -- exit loop if

condition is true END LOOP;

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PL/SQL – FOR LOOP

FOR counter IN number1..number2

LOOP

statement1;

……

END LOOP;

FOR counter IN REVERSE number1..number2

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PL/SQL – FOR LOOP ExampleFOR i IN 1..3 LOOP -- assign the values 1,2,3 to

i

sequence_of_statements -- executes three times

END LOOP;

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PL/SQL – WHILE LOOP

WHILE <condition>

LOOP

statement1;

…..

END LOOP;

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Pre-defined Errors - examples NO_DATA_FOUND

A SELECT INTO statement returns no rows, or your program references a deleted element in a nested table or an uninitialized element in an index-by table.

TOO_MANY_ROWSA SELECT INTO statement returns more than one

row. ZERO_DIVIDE

A program attempts to divide a number by zero.

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PL/SQL Error Handling example

DECLARE comm_missing EXCEPTION; -- declare own exception name VARCHAR2(20); surname VARCHAR2(20);BEGIN SELECT first_name, family_name INTO name, surname FROM AUTHORS; IF commission IS NULL THEN RAISE comm_missing; -- raise exception END IF; bonus := (salary * 0.10) + (commission * 0.15);EXCEPTION WHEN comm_missing THEN DBMS_OUPUT.PUT_LINE(‘Sorry cannot calculate bonus as there is no

commission’); WHEN TOO_MANY_ROWS THEN DBMS_OUPUT.PUT_LINE(‘To much information to store ‘);END;/

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Subprograms An ideal way of writing Writing Reusable

PL/SQL Code PL/SQL has two types of subprograms

called procedures and functions, which can take parameters and be invoked (called).

a subprogram is like a miniature program, beginning with a header followed by an optional declarative part, an executable part, and an optional exception-handling part:

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Procedure ExamplePROCEDURE award_bonus (emp_id NUMBER) IS bonus REAL; comm_missing EXCEPTION; BEGIN -- executable part starts here SELECT comm * 0.15 INTO bonus FROM emp WHERE empno = emp_id;

IF bonus IS NULL THEN RAISE comm_missing; ELSE UPDATE payroll SET pay = pay + bonus WHERE empno = emp_id; END IF; EXCEPTION -- exception-handling part starts here WHEN comm_missing THEN ... END award_bonus; ...

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Function Example

CREATE OR REPLACE FUNCTION square(

original NUMBER) RETURN NUMBER AS

original_squared NUMBER;

BEGIN

original_squared := original * original;

RETURN original_squared;

END;

/

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Packages PL/SQL lets you bundle logically related types,

variables, cursors, and subprograms into a package

The packages defines a simple, clear, interface to a set of related procedures and types.

Packages usually have two parts: a specification and a body.

The specification defines the application programming interface; it declares the types, constants, variables, exceptions, cursors, and subprograms.

The body fills in the SQL queries for cursors and the code for subprograms.

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Packages Example

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Database Triggers A database trigger is a stored subprogram

associated with a database table, view, or event. The trigger can be called once, when some event

occurs, or many times, once for each row affected by an INSERT, UPDATE, or DELETE statement.

The trigger can be called after the event, to record it or take some followup action. Or, the trigger can be called before the event to prevent erroneous operations or fix new data so that it conforms to business rules.

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Triggers Example

CREATE TRIGGER audit_sal

AFTER UPDATE OF sal ON emp

FOR EACH ROW

BEGIN

INSERT INTO emp_audit VALUES ...

END;

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Cursors

A cursor is a pointer to the private memory area allocated by the Oracle server.

There are two types of cursors:Implicit cursors: Created and managed

internally by the Oracle server to process SQL statements

Explicit cursors: Explicitly declared by the programmer

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Processing Explicit Cursors The following three commands are used to

process an explicit cursor:○ OPEN○ FETCH○ CLOSE

Every explicit cursor has the following four attributes:

○ cursor_name%FOUND○ cursor_name%ISOPEN○ cursor_name%NOTFOUND○ cursor_name%ROWCOUNT

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Cursor Example

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