Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

13
User-Defined Functions
Well, here we are already at one of my favorite topics. Long after their introduction, user-defined
functions — or UDFs — remain one of the more underutilized and misunderstood objects in SQL
Server. In short, these were awesome when Microsoft first introduced them in SQL Server 2000, and
the addition of .NET functionality back in SQL Server 2005 just added all that much more to them.
One of the best things about UDFs from your point of view is, provided you’ve done the book in
order, you already know most of what you need to write them. They are actually very, very similar
to stored procedures — they just have certain behaviors and capabilities about them that set them
apart and make them the answer in many situations.
In this chapter, we’re not only going to introduce what UDFs are, but we’re also going to take a
look at the different types of UDFs, how they vary from stored procedures (often called sprocs),
and, of course, what kinds of situations we might want to use them in. Finally, we’ll take a quick
look at how you can use .NET to expand on their power.
What a UDF Is
A user-defined function is, much like a sproc, an ordered set of T-SQL statements that are preoptimized
and compiled and can be called to work as a single unit. The primary difference between
them is how results are returned. Because of things that need to happen in order to support these
different kinds of returned values, UDFs have a few more limitations to them than sprocs do.
OK, so I’ve said what a UDF is, so I suspect I ought to take a moment to say what it is not. A
UDF is definitely NOT a replacement for a sproc — they are just a different option that offers us
yet one more form of code flexibility.
With a sproc, you can pass parameters in and also get values in parameters passed back out. You
can return a value, but that value is really intended to indicate success or failure rather than return
data. You can also return result sets, but you can’t really use those result sets in a query without
first inserting them into some kind of table (usually a temporary table) to work with them further.