Question

I would like to write this as a user defined function:

private double Score(Story s){                            DateTime now = DateTime.Now;                            TimeSpan elapsed = now.Subtract(s.PostedOn);                            double daysAgo = elapsed.TotalDays;                            return s.Votes.Count + s.Comments.Count - daysAgo;                                      } 

Is this possible to do with a UDF?

Answer 1

You can, but if you’re using SQL Server 2000, you’ll have to pass in the value of ‘now’–UDFs can’t generate any non-deterministic values themselves in SQL Server 2000.

This untested stab at it might be close:

CREATE FUNCTION dbo.GetStoryScore (     @Now DATETIME, @Posted DATETIME, @Votes INT, @Comments INT ) RETURNS FLOAT AS BEGIN    RETURN @Votes + @Comments - DATEDIFF(HOUR, @Posted, @Now)/24.0  END 

Example usage:

SELECT S.ID, dbo.GetStoryScore(GETDATE(), S.Posted, S.Votes, S.Comments) AS Score FROM Stories AS S WHERE ... 

Notes:

• The datediff is performed in hours (not days) because the integer result gives you a little more precision when you use finer units.

• I passed in all the values because I’ve found lookups within functions to be a really, really bad thing for performance.

• When referenced in SQL, don’t forget the dbo. in front of the function name.

• If you’re using SQL Server 2005, you can remove the @Now variable and use GETDATE() inline, instead.