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SQL: A Beginner s Guide
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To create a function, you must use a CREATE FUNCTION statement. The statement is similar to a CREATE PROCEDURE statement, except for a few critical differences:
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The input parameter definitions cannot include the IN keyword. A RETURNS clause must follow the parameter definitions. The clause assigns a data type to the value returned by the function. The routine body must include a RETURN statement that defines the value returned by the parameter.
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SQL Server also uses a RETURNS clause to assign a data type to the returned value, while Oracle uses a RETURN clause for the same purpose. In both cases this clause is followed by the AS keyword. Both SQL Server and Oracle use a RETURN statement in the routine body to define the value returned by the parameter.
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A function definition can include many of the elements that have been described throughout this chapter. For example, you can define local variables, create compound statements, and use conditional statements. In addition, you can define and use input parameters in the same way you define and use input parameters in procedures (except that you do not use the IN keyword). Now that you have an overview of how to create a function, let s look at an example, which is based on the IN_STOCK_CDS and PERFORMERS tables, shown in Figure 13-2. The following CREATE FUNCTION statement defines a function that returns the artist name for a specified CD, as it appears in the IN_STOCK_CDS table:
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CREATE FUNCTION CD_ARTIST ( p_Title VARCHAR(60) ) RETURNS VARCHAR(60) BEGIN RETURN ( SELECT ARTIST_NAME FROM IN_STOCK_CDS s, PERFORMERS p WHERE s.Title = p.Title AND s.Title = p_Title ); END;
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In the first line of the statement, the CD_ARTIST function and the p_Title parameter have been defined. In the next line, the RETURNS clause assigns the VARCHAR(60) data type to the value returned by the function. In the routine body, you can see that a RETURN statement has been defined. The statement includes a subquery that uses the value of the input parameter to return the name of the artist. As you can see, defining a function is not much different from defining a procedure; however, calling the function is another matter. Instead of using the CALL statement to invoke the function, you use the function as you would any of the SQL predefined functions. (You saw some of these functions in 10.) For example, suppose you want to find the name
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Creating SQL-Invoked Routines
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IN_STOCK_CDS TITLE: VARCHAR(60) STOCK: INT
PERFORMERS TITLE: VARCHAR(60) Famous Blue Raincoat Blue Court and Spark Past Light Kojiki That Christmas Feeling Patsy Cline: 12 Greatest Hits ARTIST_NAME: VARCHAR(60) Jennifer Warnes Joni Mitchell Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline
Famous Blue Raincoat 13 Blue Court and Spark Past Light Kojiki 42 22 17 6
That Christmas Feeling 8 Out of Africa Blues on the Bayou Orlando 29 27 5
After the Rain: The Soft Sounds of Erik Satie Pascal Roge Out of Africa Leonard Cohen The Best of Fundamental Blues on the Bayou Orlando John Barry Leonard Cohen Bonnie Raitt B.B. King David Motion
Figure 13-2
Using functions to retrieve values from the IN_STOCK_CDS and PERFORMERS tables
of an artist based on the CD name and you want to know what other CDs that artist has made. You can create a SELECT statement similar to the one shown in the following example to retrieve the data:
SELECT TITLE, ARTIST_NAME FROM PERFORMERS WHERE ARTIST_NAME = CD_ARTIST('Blue');
The CD_ARTIST function returns the Joni Mitchell value (the artist of the Blue CD), which is then compared to the ARTIST_NAME values. As a result, two rows are returned by the statement, as shown in the following query results:
TITLE --------------Blue Court and Spark ARTIST_NAME ------------Joni Mitchell Joni Mitchell
SQL: A Beginner s Guide
As you can see, functions help to simplify your queries by storing part of the code as a schema object (in the form of an SQL-invoked routine) and then invoking that code as necessary by calling the function as a value in your SQL statement. Functions provide you with a wide range of possibilities for returning values that make your queries less complex and more manageable.
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