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Up to this point, you ve shown us how to create SQL-invoked procedures, but not how to modify them. Is there a way to alter or delete procedures The SQL standard supports both an ALTER PROCEDURE statement and a DROP PROCEDURE statement. The ALTER PROCEDURE statement allows you to alter some of the routine characteristics of the procedure, but it does not allow you to alter the procedure body. However, the functionality supported by the ALTER PROCEDURE statement can vary so widely from one SQL implementation to another that you ll need to check the product documentation to see whether the statement is supported and what you can do with that statement. In SQL Server, for example, the ALTER PROCEDURE statement allows you to modify most aspects of the procedure, whereas the same statement in Oracle is used primarily to recompile the procedure to avoid runtime compiling (which saves on runtime overhead). However, Oracle provides the CREATE OR REPLACE PROCEDURE syntax that replaces an existing procedure (including the procedure body) or creates a new procedure if it does not yet exist.
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As for the DROP PROCEDURE statement, most implementations support this and it is usually fairly straightforward. You simply provide the name of the procedure in the statement and, depending on the SQL implementation, the RESTRICT or CASCADE keywords, as you ve seen them used in other DROP statements. Note that the same is true for the ALTER FUNCTION and DROP FUNCTION statements. Although supported by many implementations, the ALTER FUNCTION statement can vary from one product to the next, and the DROP FUNCTION statement is fairly similar across implementations.
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In addition to allowing you to pass parameters into a procedure, SQL also provides a way for you to create local variables in the procedure definition that can be used within the body of the procedure. You can think of a local variable as a type placeholder that holds a value in memory during the execution of the statements in the routine body. Once the statements are executed, the variable ceases to exist. When you define a local variable, you must first declare the variable and then set an initial value for it. You can then use that variable in the remaining block of statements. The basic syntax for defining a variable is as follows: DECLARE <variable name> <data type>; As you can see, the syntax is very straightforward. You must provide a name for the variable and assign a data type. Once you ve declared the variable, you must then assign a value to it before it can be referenced. (However, some implementations automatically assign a null value to variables as they are defined.) You can use the SET statement to assign a value to a variable, which has the following syntax: SET <variable name> = <value expression>; In this statement, you must first provide the variable name and then provide the value, which can be any sort of value expression, such as a number, a character string, or a subquery. After you ve declared the variable and assigned it a value, you re ready to use the variable in your routine body. The best way to illustrate this is to show you an example of a procedure that uses a variable. For this example, we ll again use the CD_INVENTORY table, shown in Figure 13-1. The following statement creates a procedure that retrieves CD information for a specific music type:
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CREATE PROCEDURE CD_AMOUNT ( IN p_Type_ID CHAR(4) ) BEGIN DECLARE v_Amount INT; SET v_Amount = ( SELECT AVG(CD_STOCK) FROM CD_INVENTORY ); SELECT CD_TITLE, CD_STOCK
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