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SQL: A Beginner s Guide
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FROM CD_INVENTORY WHERE CD_TYPE_ID = p_Type_ID AND CD_STOCK < v_Amount; END;
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Let s go through this statement line by line. In the first line, we create a procedure named CD_AMOUNT and an input parameter named p_Type_ID. The second line contains the keyword BEGIN. The BEGIN keyword is paired with the END keyword in the last line. Together they enclose a block of statements that are processed as a unit. We ll take a closer look at the BEGIN...END block later in the Working with Control Statements section. The third line of the procedure definition includes a DECLARE statement that declares the v_Amount variable, which is defined with the INT data type. The next two lines use a SET statement to assign an initial value to the parameter. This value is derived from a subquery that finds the average for all the CD_STOCK values. The average in this case is about 13. In the next four lines of the procedure definition, a SELECT statement retrieves data from the CD_INVENTORY table based on the values supplied by the parameter and variable. Once you ve created your procedure, you can execute it by using a CALL statement and providing a value for the parameter, as shown in the following example:
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CALL CD_AMOUNT('NEWA');
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When the procedure is processed, it uses the NEWA value from the parameter and the CD_STOCK average from the variable in the SELECT statement defined in the procedure definition. It would be similar to executing the following statement:
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SELECT FROM WHERE AND CD_TITLE, CD_STOCK CD_INVENTORY CD_TYPE_ID = 'NEWA' CD_STOCK < 13;
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This SELECT statement, like the procedure itself, will return the following query results:
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CD_TITLE ---------Past Light Kojiki CD_STOCK -------6 10
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Notice that both rows contain CD_STOCK values less than the average amount (13) and both are New Age CDs. You re not limited to only one variable in a procedure definition. You can create a DECLARE statement for each variable that you want to include. You can also include multiple variables in one statement, if those variables are assigned the same data type. For example, suppose you want to declare several variables with an INT data type, as shown in the following DECLARE statement:
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DECLARE Var1, Var2, Var3 INT;
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13:
Creating SQL-Invoked Routines
This statement declares the Var1, Var2, and Var3 variables, and each one is assigned the INT data type. Once you assign initial values to the variables, you can use them in the routine body in the same way as any other local variables.
Work with Control Statements
When the SQL/PSM standard was released in 1996, it included not only language that supported SQL-invoked routines, but language that could be used within those routines to make them more robust. Such characteristics as grouping statements into blocks and looping statements so that they could be executed multiple times behavior traditionally associated with procedural type languages made procedures and functions even more valuable to users needing to access and manipulate data in their databases. The SQL:2006 standard refers to these new language elements as control statements because they affect how you can control data in SQL-invoked routines. In this section, we ll look at several of these control statements, including those that allow you to group statements into a block, create conditional statements, and set up statements into a loop.
Create Compound Statements
The most basic of the control statements is the compound statement, which allows you to group statements into a block. The compound statement starts with the BEGIN keyword and finishes with the END keyword. Everything between the two keywords is part of the block. The compound statement is made up of one or more individual SQL statements, which can include statements such as DECLARE, SET, SELECT, UPDATE, INSERT, DELETE, or other control statements. You ve already seen an example of a compound statement in the preceding CREATE PROCEDURE statement that defines the CD_AMOUNT procedure. (This is the example shown in the Add Local Variables to Your Procedures section.) If you take another look at that example, you ll see that the procedure definition includes a compound statement. As you would expect, it starts with the BEGIN keyword and finishes with the END keyword. The block created by these keywords includes a DECLARE statement, a SET statement, and a SELECT statement. Notice that each statement is terminated with a semicolon. Although the BEGIN...END statement is considered one statement, the statements enclosed in the keywords are individual statements in their own right.
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