Transcript Advanced PL/SQL Programming

Chapter 4B:
More Advanced PL/SQL Programming
Monday 2/23/2015
© Abdou Illia
MIS 4200 - Spring 2015
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
2
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
3
• 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
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
5
IF/THEN/ELSE
Nested IF/THEN/ELSE
IF/ELSIF
8
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
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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
INSERT INTO Student (s_first)
VALUES (curr_first_name);
WHERE s_first = curr_first_name;
10
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
– LOOP…EXIT WHEN
– Cursor FOR loop
- WHILE…LOOP
- Numeric FOR loop
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The LOOP...EXIT Loop
SQL statement
CREATE TABLE count_table
(counter NUMBER(2));
• Pretest or posttest
• Syntax:
Syntax
LOOP
[program statements]
IF condition THEN
EXIT;
END IF;
[additional program
statements]
END LOOP;
The LOOP...EXIT WHEN Loop
Syntax (posttest loop)
LOOP
program statements
EXIT WHEN condition;
END LOOP;
The WHILE...LOOP
Syntax
• WHILE…LOOP
is a Pretest loop
Q: Why the SELECT
* FROM count_table
shows only counter 6
to 10?
WHILE condition LOOP
program statements
END LOOP;
The Numeric FOR Loop
• Does not require explicit counter increment
Syntax
FOR counter_variable IN start_value .. end_value
LOOP
program statements
END LOOP;
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_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
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 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
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
• 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
– Constraint violation
- inserting incompatible data
- 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;
• Handling error procedure depends the type of exception:
– Predefined exception
– User-defined exception
- undefined 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
Loc_id 60 doesn’t exist in LOCATION
• The ORA-02291 exception is not predefined.
• Need to explicitly declare the exception and write a handler
Creating an exception handler
Syntax
DECLARE
e_exception_name EXCEPTION;
PRAGMA EXCEPTION_UNIT(e_exception_name, -Oracle_error_code);
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