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CHAPTER 7 LEVERAGING DATABASE OBJECTS THAT ENCAPSULATE T-SQL
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more situations than the code inside a view. The number of situations in which you can reuse an IF user-defined function is the number of legitimate, distinct values of the parameters for the IF userdefined function. This section introduces you to programming techniques for user-defined functions by presenting samples of FN and IF user-defined functions with and without parameters. You ll learn how to create and drop these database objects. In addition, you ll learn how to invoke them after you create them. This section does not cover TF user-defined functions because they are substantially more complicated than the other types, and many of their special capabilities can readily be accomplished by standard T-SQL programming outside of a user-defined function or with other database objects.
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Creating and Using FN User-Defined Functions
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You can use a CREATE FUNCTION statement to create any of the three types of user-defined functions. After you create a new FN user-defined function, you can use it in the same way as a built-in function. One distinction between FN user-defined functions and built-in functions is that you must specify the schema for the FN user-defined function when you invoke it. For example, use dbo.FNFunctionName() instead of BuiltinFunctionName(). The samples in this section reside in FNUserDefinedFunctions.sql.
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As with CREATE statements for other SQL Server objects, you cannot create a new instance of an FN user-defined function with the name of an existing FN user-defined function. Therefore, you need to insure that the name for your new user-defined function does not already exist. The following code excerpt establishes the AdventureWorks database as the database context and then searches for a previous instance of an FN user-defined function in the sys.objects view. The code sample designates the type of object by specifying FN for the type column value in the WHERE clause. Next, the code designates a search of the dbo schema by setting schema_id equal to 1 in the WHERE clause. Finally, the sample designates the name of the FN user-defined function through the name column value for the view. If the FN user-defined function exists, the code simply drops the previous version. You can write code to archive prior versions if you prefer. The sp_rename system-stored procedure can assist with this task, but you may need to perform some object management if you want to maintain more than just a single backup version of a database object. USE AdventureWorks GO IF EXISTS( SELECT * FROM sys.objects WHERE type = 'FN' AND schema_id = SCHEMA_ID('dbo') AND name = 'ufnCntCustomers') DROP FUNCTION dbo.ufnCntCustomers GO The next code excerpt demonstrates the syntax for an FN user-defined function that returns an aggregate value. The aggregate value is the count of the rows in the Customer table of the Sales schema. The designation of the owner (dbo) after CREATE FUNCTION as a name qualifier is optional,
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CHAPTER 7 LEVERAGING DATABASE OBJECTS THAT ENCAPSULATE T-SQL
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but it is good programming practice because it explicitly reminds you of the schema for your new FN user-defined function. Every CREATE FUNCTION statement for an FN function must have a RETURNS clause, an AS keyword, and one or more lines of code within a BEGIN END block. At least one statement within the BEGIN END block should be a RETURN statement because this statement specifies a return value from the function. The RETURNS clause denotes the type of scalar value that a function returns. You can return any scalar data type from an FN user-defined function, except for text, ntext, image, or timestamp. The AS keyword marks a transition from the header area to the body area. The header area includes the CREATE FUNCTION phrase as well as the RETURNS clause. The body of the FN userdefined function is the code within the BEGIN END block. You can have multiple lines of code within a BEGIN END block, but even if your FN userdefined function has just a single line of code, it still requires a BEGIN END block. CREATE FUNCTION dbo.ufnCntCustomers() RETURNS int AS BEGIN RETURN (SELECT COUNT(*) FROM Sales.Customer) END GO After you create an FN user-defined function, the next step is to use it. The following listing illustrates some code for invoking the ufnCntCustomers user-defined function in the dbo schema. In particular, notice the specification of a schema name qualifier before the function name. Also, you must add parentheses after the function even if the function has no parameters. The listing also shows how to drop the FN user-defined function with the DROP FUNCTION statement. In the current database context, this step restores the AdventureWorks database to its initial state. The total count of customers in the AdventureWorks database is 19,185. SELECT dbo.ufnCntCustomers() '# of Customers' GO DROP FUNCTION ufnCntCustomers GO # of Customers -------------19185
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