Pl sql

More Advanced PL/SQL Programming

Lesson B ObjectivesAfter completing this lesson, you should be able

to:• Create PL/SQL decision control structures• Use SQL queries in PL/SQL programs• Create loops in PL/SQL programs• Create PL/SQL tables and tables of records• Use cursors to retrieve database data into

PL/SQL programs• Use the exception section to handle errors in

PL/SQL programs

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IF/THEN• Decision control structures

– Alter order in which statements execute– Based on values of certain variables

• Syntax:IF condition THEN commands that execute if condition is TRUE;END IF;

• Condition – Expression evaluates to TRUE or FALSE– If TRUE commands execute

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• Question: What two lines do you need to change to make the program display Today is not Friday when the current day is different than Friday?

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IF/THEN/ELSE• Syntax:

IF condition THEN commands that execute if condition is TRUE;ELSE commands that execute if condition is FALSE;END IF;

• Evaluates ELSE command if condition FALSE

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Nested IF/THEN/ELSE• Placing one or more IF/THEN/ELSE statements

within program statements that execute after IF or ELSE command

• Important to properly indent program lines

IF/THEN/ELSE

Nested IF/THEN/ELSE

IF/ELSIF

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Logical Operators AND, OR, and NOT• Create complex expressions for decision control structure

condition• AND: Expressions on both sides of operator must be true

for combined expression to be TRUE• OR: Expressions on either side of operator must be true

for combined expression to be TRUE• Order of evaluation (precedence):

– NOT– AND– OR

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• Parentheses can be used to override precedence and force the program to evaluate the OR first

Using SQL Queries in PL/SQL Programs• Use SQL action query

– Put query or command in PL/SQL program– Use same syntax as the syntax used to execute query or

command in SQL*Plus– Can use variables instead of literal values like ‘Tammy’• To specify data values

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INSERT INTO Student (s_first)VALUES (curr_first_name);

WHERE s_first = curr_first_name;

Changing the values one by one and adding them to the table requires a lot of coding. What is the best way of handling this kind of repetitive job in programming?

Loops• Systematically executes program statements• Periodically evaluates exit condition to

determine if loop should repeat or exit• Pretest loop– Evaluates exit condition before any program

commands execute• Posttest loop – Executes program commands before loop

evaluates exit condition for first time• PL/SQL has 5 types of loop structures:

– LOOP…EXIT - WHILE…LOOP– LOOP…EXIT WHEN- Numeric FOR loop– Cursor FOR loop

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The LOOP...EXIT Loop

• Pretest or posttest• Syntax:

CREATE TABLE count_table(counter NUMBER(2));

SQL statement LOOP [program statements] IF condition THEN EXIT; END IF; [additional program statements]END LOOP;

Syntax

The LOOP...EXIT WHEN LoopLOOP program statements EXIT WHEN condition;END LOOP;

Syntax (posttest loop)

The WHILE...LOOP• WHILE…LOOP is

a Pretest loopWHILE condition LOOP program statementsEND LOOP;

Syntax

Q: Why the SELECT * FROM count_table shows only counter 6 to 10?

The Numeric FOR Loop• Does not require explicit counter increment

FOR counter_variable IN start_value .. end_valueLOOP program statementsEND LOOP;

Syntax

Cursor• A pointer to memory location on database

server • Used to: – Retrieve and manipulate database data in

PL/SQL programs

• Types:– Implicit cursor– Explicit cursor

Implicit Cursors• Context area

– A memory location created by INSERT, UPDATE, DELETE, or SELECT

– Contains information about query (# rows, etc.)

• Active set– Set of data rows that query retrieves when a SELECT query is

issued

• Implicit cursor – A pointer to the context area– Called so, because you do not need to write code to

explicitly create the cursor or retrieve its values– Used to assign output of SELECT query to PL/SQL program

variables when query will return only one record*

* Error occurs if query returns no records or more than one record

Implicit Cursors (continued)• To retrieve data using implicit cursor in

PL/SQL, you add an INTO clause to the SELECT query

• Syntax:SELECT field1, field2, ...INTO variable1, variable2, ...FROM table1, table2, ...WHERE join_conditionsAND search_condition_to_retrieve_1_record;

• Variables must be declared in Declaration section

• Variables must have same data types as fields• To avoid errors, %TYPE reference data type

should be used

Implicit Cursors (continued)

Explicit Cursors• Retrieve and display data in PL/SQL programs

for query that might – Retrieve multiple records– Return no records at all

• Must explicitly write the code to– Declare cursor– Open cursor– Fetch data rows– Close cursor

Explicit Cursors (continued)

• Declare explicit cursor syntax:– CURSOR cursor_name IS select_query;

• Open explicit cursor syntax:– OPEN cursor_name;

• Fetch values using LOOP…EXIT WHEN loop:LOOP FETCH cursor_name INTO variable_name(s);EXIT WHEN cursor_name%NOTFOUND;

• Close cursor syntax:– CLOSE cursor_name;

Note: When the cursor is declared, system doesn’t check syntax error in the query. It creates the memory structure to store the active set. The PL/SQL interpreter checks for error and interprets the query when opening the cursor

Q: At this point, what is the value for current_bldg_code?

• Using %ROWTYPE variable to display explicit cursor values

Using a cursor and a single variable to retrieve multiple fields values

Handling Runtime Errors in PL/SQL Programs

• Runtime errors– Occur when an exception (unwanted event) is raised– Cause program to fail during execution

• Possible causes (exceptions):– Division by zero - inserting incompatible data– Constraint violation - retrieving 0/several rows with implicit cursor

• Exception handling– Programmers place commands in EXCEPTION section

• Handle exception options– Correct error without notifying user of problem– Inform user of error without taking corrective action

• After exception handler executes– Program ends

DECLARE variable declarationsBEGIN program statementsEXCEPTION error-handling statementsEND;

• Handling error procedure depends the type of exception:– Predefined exception - undefined exception– User-defined exception

Predefined Exceptions• Most common errors that occur in programs• PL/SQL language:

– Assigns exception name – Provides built-in exception handler for each predefined exception

• System automatically displays error message informing user of nature of problem

Exception Handler Syntax• Can create exception handlers to display alternate

error messages

Using the WHEN OTHERS exception• The SQLERRM built-in function is used to handle other exception• To use the SQLERRM function, you must

– Declare a VARCHAR2 variable– Assign the declared variable the error’s text and code

Undefined Exceptions• Less common errors • Do not have predefined names• Must explicitly declare exception in program’s

declaration section• Associate new exception with specific Oracle

error code• Create exception handler in exception section – Using same syntax as for predefined exceptions

Example of undefined exception

• The ORA-02291 exception is not predefined.• Need to explicitly declare the exception and write a handler

Loc_id 60 doesn’t exist in LOCATION

User-defined Exceptions• Used to handle an exception that – Does not raise Oracle runtime error– But requires exception handling to

• Enforce business rules or • Ensure integrity of database

• Example:– Internal Northwoods’ rule is “Users can delete

row from the ENROLLMENT table only if s_grade is NULL”

– Trying to delete a delete an ENROLLMENT row where the s_grade is not NULL will raise an exception that needs to be handled