Unrolling the loop is an interesting idea, I will have a play with that in a minute to see if it works the same for me. I think I prefer the loop though - it's neater to my mind.

Handling NULL and -2147483648 is another good point (though the original test rig can produce neither value, in my defence).

The point I was looking to make is that expression evaluation in T-SQL is interpreted and therefore quite slow.

It is amazing that passing a value out to CLR and back is faster, even without working hard to find an optimal .NET algorithm.

Anyway, updated code to handle all values and NULL below:

CREATE ASSEMBLY Test

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WITH PERMISSION_SET = SAFE;

GO

CREATE FUNCTION dbo.SumDigits

(@Input integer)

RETURNS integer

AS EXTERNAL NAME Test.UserDefinedFunctions.SumDigits;

Source:

[font="Courier New"]public static SqlInt32 SumDigits(SqlInt32 Input)

{

    if (Input.IsNull)

    {

        return SqlInt32.Null;

    }

    int sum = 0;

    for (int n = Input.Value; n != 0; sum += n % 10, n /= 10) ;

    return new SqlInt32(sum >= 0 ? sum : -sum);

}

[/font]