Ajay,
Just some advice for you
The case statement that you've given will still return multiple rows - the sum should be round the whole case statement rather than just the @Select statement.
You would end up with a total from the @Select plus however many 0s for the number of rows where the code is not A, B or C.
While the projection set as sum will still give you the correct answer in webi, you would be bringing back far more rows that you intended to.
Regards,
Mark