User defined functions are very simular to stored procedures. Rather then returning recordsets, they can only return variables. They are also limited to the types of variables you may return. For example - text, ntext, and image datatypes can not be returned.

The magic of user defined functions is that you can use them in SELECT, UPDATE, DELETE, & SET statments. You can use them in triggers and other places. Somehow I get the idea that you are scratching your head still asking "What is it?". Let me show you a small example.

SELECT UserID, dbo.FullName(UserID) FROM Users

In this example, the function is called FullName. As you may have guessed, the function returns the full name of the user within the record.

Glad you asked. When you open your database in the SQL Server Enterprise Manager, one of the nodes available will state "User Defined Functions". I usually do a right-mouse-click and click on

A default template may appear ...

This function is not valid nor helpful. It doesn't even pass when you click on the button "Check Syntax". It took me a good amount of research along with trial & error before I got the gist of what was going on.

Right O! Lets go with creating the "FullName" function. Here is my code:

CREATE FUNCTION [dbo].[FullName] (@UserID int)
RETURNS varchar(255)
AS 
    BEGIN 
    	Select
    		@FullName = @FirstName + ' ' + @LastName
    	FROM
    		[Users]
    	WHERE
    		[UserID] = @UserID
    	Return(@FullName)
    END
    

So first, you can see that the parameter list has been changed to pass a Integer variable called UserID. You can add more parameters as long as you seperate them with a comma.

Next we see that the return type, and the parethesies around it has been replaced with a datatype "varchar(255)". You are not limited to varchars alone. Just about any type of datatype is available except cursors, records, and datatypes with pointers (text, ntext, image, etc.).

If you look at the very bottom, you will see "Return(@FullName)". This is where we return the results to the query that called the function.

And within the BEGIN and END statements, we have the body of the function.

The demonstration here is very inefficient. It is here only as a reference to get you started. Defining your own functions allows more complex business logic & formatting to be possible. Consider the ability to pass in a piece of text of 4000 characters long and inserting HTML links on specific keywords. Another idea would be to perform lookups on multiple tables and return a value that best suites your query.