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SQL: A Beginner s Guide
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The condition statement in the preceding example shows only two conditions and courses of action: the condition/action defined in the IF clause and the condition/ action defined in the ELSE clause. What if you want to include more conditions The SQL:2006 standard supports more than two condition/action constructions in a conditional statement. If more than two are needed, you treat the IF clause and the ELSE clause as shown in the example. The additional conditions are inserted between the two clauses by adding an ELSE IF clause or an ELSEIF clause. The syntax for this would be as follows:
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IF <condition> THEN <action> ELSE IF <condition> THEN <action> ELSE <action>
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The exact way you implement the third condition/action depends on your implementation. In addition, not all implementations support ELSEIF, and some use the ELSIF keyword. As always, be sure to refer to your product documentation.
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Now let s take a look at another type of control statement the looping statement. SQL actually supports several types of looping statements. We ll be looking at two of them: the LOOP statement and the WHILE statement, both of which perform similar functions. The LOOP statement uses the LOOP and END LOOP keywords to enclose a block of statements that are executed repeatedly until the loop is explicitly ended, usually through the use of the LEAVE keyword. Note that Oracle uses the EXIT keyword instead of LEAVE, and SQL Server does not support the LOOP statement. Let s take a look at an example to illustrate how this looks. Once again using the tables in Figure 13-1, we ll use a LOOP statement to update the CD_INVENTORY table.
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If you created and tested the CD_AMOUNT procedure in the preceding example, assume that the CD_INVENTORY table has been returned to its original condition shown in Figure 13-1 and that no data has been modified.
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In the following procedure definition, I include a LOOP statement that continues to update the CD_STOCK column until it reaches an amount greater than 14:
CREATE PROCEDURE UPDATE_STOCK ( IN p_Title CHAR(20) ) BEGIN DECLARE v_Amount INT;
13:
Creating SQL-Invoked Routines
SET v_Amount = ( SELECT CD_STOCK FROM CD_INVENTORY WHERE CD_TITLE = p_Title ); Loop1: LOOP SET v_Amount = v_Amount + 1; UPDATE CD_INVENTORY SET CD_STOCK = v_Amount WHERE CD_TITLE = p_Title; IF v_Amount > 14 THEN LEAVE Loop1; END IF; END LOOP; END;
In this statement, the loop is first assigned a name (Loop1:), which is sometimes called a statement label. You must include a colon with the name when you first assign it. Next you create your loop block, which begins with the LOOP keyword and finishes with the END LOOP keywords. Within the block are SET and UPDATE statements. These two statements are executed until the loop is terminated. Notice that the CD_STOCK value is increased by an increment of 1 each time the statements in the loop are executed. These two statements are followed by an IF statement, which specifies the condition in which the loop is terminated. If the value for the v_Amount variable exceeds 14, then the loop is terminated (LEAVE Loop1). The IF statement is then ended with the END IF keywords.
NOTE
If you do not include the IF statement within the loop (with the LEAVE termination operator), the loop will continue to increase the CD_STOCK value until it fills all available storage or some other event terminates the operation. This is a common programming error known as an infinite loop.
You can then invoke the procedure by providing the procedure name and a value for the parameter. For example, suppose you want to update the Fundamental row in the CD_ INVENTORY table. You can invoke the procedure with the following CALL statement:
CALL UPDATE_STOCK('Fundamental');
When the procedure is executed, a value of 1 is repeatedly added to the CD_STOCK column until the value reaches 15, and then the loop is terminated. You can receive the same results more elegantly by using a WHILE statement. In the following example, I modified the UPDATE_STOCK procedural definition by replacing the LOOP statement with a WHILE statement:
CREATE PROCEDURE UPDATE_STOCK ( IN p_Title CHAR(20) ) BEGIN DECLARE v_Amount INT;
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