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In some SQL implementations, the compound statement might not be necessary under certain circumstances. In these cases, the semicolon terminator might be enough to signal to the implementation that one statement has ended and another has begun. Even those implementations that don t require the semicolon, such as SQL Server, will sometimes process multiple statements as a block even if the BEGIN...END construction has not been used. When the implementation reaches the end of one statement, it simply continues on to the next. However, as a general rule, you should use the compound construction to keep together those statements that should be processed as a unit. When you don t use it, you can sometimes experience unpredictable behavior, depending on the implementation.
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You can use the compound statement wherever you need to keep SQL statements together. That means that they can be embedded within other compound statements or within other types of control statements. The BEGIN and END keywords do not affect how data might be passed from one statement to the next, as in the case of parameters. The good news about compound statements and the BEGIN...END construction is that they re supported by most SQL implementations, although there can be slight variations from one product to the next, in terms of the specifics of how they re implemented. Be sure to check the product documentation when using these statements.
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The next type of control statement we ll look at is the conditional statement. This statement determines whether a statement (or series of statements) is executed based on whether a specified condition evaluates to true. The statement uses the IF, THEN, and ELSE keywords to establish the conditions and define the actions to take: if the condition is met, then the SQL statement is executed, or else another action is taken.
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The conditional statement is sometimes referred to as an IF statement, an IF...ELSE statement, an IF...END IF statement, or an IF...THEN...ELSE statement.
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Let s take a look at an example that uses a conditional statement to define different courses of action, depending on the condition. In the following procedure definition, I modified the routine body of the CD_AMOUNT procedure (which we used in the preceding example) to include a conditional statement:
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CREATE PROCEDURE CD_AMOUNT ( IN p_Type_ID CHAR (4) ) BEGIN DECLARE v_Amount INT; SET v_Amount = ( SELECT SUM(CD_STOCK) FROM CD_INVENTORY WHERE CD_TYPE_ID = p_Type_ID ); IF v_Amount < 20 THEN SELECT CD_TITLE, CD_STOCK FROM CD_INVENTORY WHERE CD_TYPE_ID = p_Type_ID; ELSE SELECT CD_TITLE, CD_STOCK FROM CD_INVENTORY; END IF; END;
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Notice that the BEGIN...END block now includes an IF...END IF statement. The IF clause introduces the statement and sets up the condition. For the condition to evaluate to true, the value of the v_Amount variable must be less than 20. If the condition evaluates to true, the first SELECT statement is executed. This is the SELECT statement that follows the THEN keyword.
13:
Creating SQL-Invoked Routines
If the condition is false, then the second SELECT statement is executed. This is the statement that follows the ELSE keyword. To sum this all up, if v_Amount is less than 20, the CD_TITLE and CD_STOCK values from the CD_INVENTORY table are returned for those rows that contain the Type ID (column CD_TYPE_ID) specified by the p_Type_ID parameter. If v_Amount is not less than 20, the CD_TITLE and CD_STOCK values for all rows in the CD_INVENTORY table are returned. Once you create your procedure, you can invoke it by using a CALL statement, as you have for previous procedures. For example, if you want to return New Age (NEWA) CDs, you can use the following CALL statement:
CALL CD_AMOUNT( NEWA );
This statement will return both New Age rows: Past Light and Kojiki. This is because the total number of New Age CDs (16) is less than 20, so the first SELECT statement is executed. If you had specified the Classic Pop category (CPOP) when you invoked the CD_AMOUNT procedure, all rows would have been returned. This is because the total number of Classic Pop CDs (28) exceeds 20. As a result the IF condition would not be met, so the ELSE statement would be executed. If you want to create a conditional statement that includes more than one SQL statement in either the IF clause or the ELSE clause, you can enclose those statements in a control statement. For example, if we add an UPDATE statement to the condition in the preceding example and use a control statement to enclose the UPDATE and SELECT statements, your procedure definition will look like the following:
CREATE PROCEDURE CD_AMOUNT ( IN p_Type_ID CHAR (4) ) BEGIN DECLARE v_Amount INT; SET v_Amount = ( SELECT SUM(CD_STOCK) FROM CD_INVENTORY WHERE CD_TYPE_ID = p_Type_ID ); IF v_Amount < 20 THEN BEGIN UPDATE CD_INVENTORY SET CD_STOCK = CD_STOCK + 1 WHERE CD_TYPE_ID = p_Type_ID; SELECT CD_TITLE, CD_STOCK FROM CD_INVENTORY WHERE CD_TYPE_ID = p_Type_ID; END; ELSE SELECT * FROM CD_INVENTORY; END IF; END;
The compound statement groups the two statements into one block of code. This way, the tables will be updated and the results of the update will be displayed in your query results.
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