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Steve Jones is the editor of and visits a wide variety of data related topics in his daily editorial. Steve has spent years working as a DBA and general purpose Windows administrator, primarily working with SQL Server since it was ported from Sybase in 1990. You can follow Steve on Twitter at

Deterministic Functions

Do you know what determinism is? It's something that comes up periodically in Books Online as various SQL objects require deterministic functions. Things like indexes views.

I ran across a question on this for our ASK site, and thought it was interesting. I started to reply, then had to double check myself and make sure by looking things up in Books Online. I was right, though I'm not sure I'd have been able to actually classify all functions correctly.

For example, RAND can be either deterministic or non-deterministic, depending on how it's used. If you have a seed value, it's deterministic. CAST and CONVERT are usually deterministic but it depends on the data type. There are some good exceptions spelled out in this BOL entry: Deterministic and Nondeterministic Functions

I knew that deterministic meant you could determine the output given the input, and that these are "predictable" functions. However I didn't realize that somethings, like GetDATE() wouldn't fit. It always returns the current time/date, but it's not based on input, or the database, so it doesn't count. It's a non-deterministic function.

Non-deterministic functions like the @@ ones (@@Connections, @@idle, etc.) are listed, and I'm not sure I'd have thought of them off the top of my head, but it was good to read them in the article. Next time I need to I should remember, or at least be able to easily scan code and decide if it's deterministic or not.


Posted by Robert Davis on 28 October 2009

Getdate() is non-deterministic because the value it outputs is affected by outside forces. For example, I can change the value returned by Getdate() by changing the time on the server.

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