Question

I have a user defined function (e.g. myUDF(a,b)) that returns an integer.

I am trying to ensure this function will be called only once and its results can be used as a condition in the WHERE clause:

SELECT col1, col2, col3, 
       myUDF(col1,col2) AS X
From myTable
WHERE x>0

SQL Server tries to detect x as column, but it’s really an alias for a computed value.

How can you re-write this query so that the filtering can be done on the computed value without having to execute the UDF more than once?

Answer 1

If you are using SQL Server 2005 and beyond, you can use Cross Apply:

Select T.col1, T.col2, FuncResult.X
From Table As T
    Cross Apply ( Select myUdf(T.col1, T.col2) As X ) As FuncResult
Where FuncResult.X > 0