L9 L10- Server Side Programming

Database Systems Server side Database Programming using PL/SQL Version 1.1

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What is PL/SQL? 

SQL  Structured language to access database  ANSI Standard  Declarative ○ 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 data Also stores and executes Procedural code!

Advantages Results 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

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Is there a problem?  Performance impact due increased network traffic 

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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 block  Contains variables, code and error handler

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

nothing!

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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

Optional

END; 7

PL/SQL - EXCEPTIONs 

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

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EXCEPTION HANDLER  Code that executes when EXCEPTION occurs  Makes the code more robust

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Oracle Server has many pre-defined errors  no_data_found, value_error, too_many_rows, others

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PL/SQL - Anonymous Block DECLARE 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 trigger type 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 Character DECLARE 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

DECLARE

author_id AUTHORS.ID%TYPE; PL/SQL variable ‘author_id’ is of same datatype which 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 Example DECLARE father_name VARCHAR2(20):='Patrick'; date_of_birth DATE:='20-Apr-1972'; BEGIN DECLARE 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 operands Arithmetic Operators ○ **, *, /, +, Comparison Operators ○ =, <>, !=, <, >, <=, >=, LIKE, BETWEEN, IN, IS

NULL Logical Operators ○ AND, OR, NOT

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String Operator – Concatenation using ‘||’ 15

PL/SQL – NULL   

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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 appropriate

NVL (<expression>, )

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PL/SQL – SELECT sample DECLARE 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?  Example SET 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-ELSIF CASE

 Repeated

execution until some condition

 LOOP-END LOOP, FOR-LOOP-END

LOOP WHILE-LOOP-END LOOP EXIT WHEN  Jump

to code section

GOTO 19

PL/SQL – Conditional Execution IF THEN statement1; statement2; ….. END IF;  can be evaluated to TRUE, FALSE or NULL – statement1, statement2 etc., are executed only if it evaluates to TRUE 20

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 THEN statement1; statement2; ….. ELSE statement3; statement4; ….. END IF; 22

PL/SQL IF-THEN-ELSIF IF THEN statement1; ….. ELSIF 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; 24

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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; 26

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; 27

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 Example FOR 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 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.

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TOO_MANY_ROWS  A SELECT INTO statement returns more than one

row.

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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; / 32

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 Example PROCEDURE 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    

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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. 36

Packages Example

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Database Triggers  

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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

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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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