The calls to UDFs are very expensive. If UDF is called for each record than it'll slow down query significantly. The first one guaranties that UDF will be called only 2 times. The second case depends on how smart SQL Query optimizer is.

>I've inherited a system with SQL stored procedures and functions. I want to clarify my understanding.
>
>Would this ...
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>select @startdate = dbo.myfunction(@startdate)
>select @enddate = dbo.myfunction(@enddate)
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>SELECT * from table where datefield between @startdate and @enddate
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>be better than this...
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>SELECT * from table where datefield between dbo.myfunction(@startdate) and dbo.myfunction(@enddate)