1. Chapter 4B: More Advanced PL/SQL Programming
Monday 2/23/2015
© Abdou Illia MIS Spring 2015

2. Lesson B Objectives
After 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

3. 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

4. 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? 4

5. IF/THEN/ELSE Nested 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
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

6. IF/THEN/ELSE

7. Nested IF/THEN/ELSE

8. IF/ELSIF

9. 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
Parentheses can be used to override precedence and force the program to evaluate the OR first

10. 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
INSERT INTO Student (s_first) VALUES (curr_first_name);
WHERE s_first = curr_first_name;

11. 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?

12. 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

13. The LOOP...EXIT Loop Pretest or posttest Syntax: Syntax SQL statement
[program statements]
IF condition THEN
EXIT;
END IF;
[additional program statements]
END LOOP;
Syntax
CREATE TABLE count_table (counter NUMBER(2));
SQL statement
Pretest or posttest
Syntax:

14. Syntax (posttest loop)
The LOOP...EXIT WHEN Loop
LOOP
program statements
EXIT WHEN condition;
END LOOP;
Syntax (posttest loop)

15. The WHILE...LOOP WHILE…LOOP is a Pretest loop WHILE condition LOOP
program statements
END LOOP;
Syntax
WHILE…LOOP is a Pretest loop
Q: Why the SELECT * FROM count_table shows only counter 6 to 10?

16. The Numeric FOR Loop Does not require explicit counter increment
FOR counter_variable IN start_value .. end_value
LOOP
program statements
END LOOP;
Syntax

17. Cursor A pointer to memory location on database server Used to: Types:
Retrieve and manipulate database data in PL/SQL programs
Types:
Implicit cursor
Explicit cursor

18. Implicit Cursors Context area Active set Implicit cursor
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

19. 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_conditions
AND 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

20. Implicit Cursors (continued)

21. 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

22. 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 errors 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

23. Q: At this point, what is the value for current_bldg_code?
The declared variable is used to retrieve the cursor content and to display

24. Using %ROWTYPE variable to display explicit cursor values
Using a cursor and a single variable to retrieve multiple fields values

25. Handling Runtime Errors in PL/SQL Programs
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 declarations BEGIN program statements EXCEPTION error-handling statements END;

26. Handling error procedure depends the type of exception:
Predefined exception - undefined exception
User-defined exception

27. 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

28. Exception Handler Syntax
Can create exception handlers to display alternate error messages

30. 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

31. 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

32. Example of undefined exception
Loc_id 60 doesn’t exist in LOCATION
The ORA exception is not predefined.
Need to explicitly declare the exception and write a handler

33. Creating an exception handler
DECLARE e_exception_name EXCEPTION; PRAGMA EXCEPTION_UNIT(e_exception_name, -Oracle_error_code);
Syntax

34. 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