Lesson B Objectives IF/THEN. Chapter 4B: More Advanced PL/SQL Programming

Transcription

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

2 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 2

3 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 9 program to evaluate the OR first 3

4 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 - WHILE LOOP LOOP EXIT WHEN - Numeric FOR loop Cursor FOR loop 12 4

5 Pretest or posttest Syntax: The LOOP...EXIT Loop SQL statement CREATE TABLE count_table (counter NUMBER(2)); 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; WHILE LOOP is a Pretest loop The WHILE...LOOP Syntax WHILE condition LOOP program statements END LOOP; Q: Why the SELECT * FROM count_table shows only counter 6 to 10? 5

6 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 Context area Implicit Cursors 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 6

7 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 7

8 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 8

9 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 declarations BEGIN program statements EXCEPTION error-handling statements END; 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 9

10 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 10

11 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 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); 11

12 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 12