Stored Procedure with conditional results

Question

I want to write a stored procedure that works something like this:

SELECT * from T where T.A = @a and T.B = @b

if that returns rows, return those rows, if not, return

SELECT * from T where T.A = @a and T.B IS NULL

Edit:

It feels that there should be a way to create a procedure such that it runs the first query once and runs the second query only if necessary.

End Edit.

The best I could manage was the follow, which (in theory) runs the first query twice, unless maybe its cached:

IF EXISTS (SELECT * from T where T.A = @a and T.B = @b) THEN
    SELECT * from T where T.A = @a and T.B = @b
ELSE
    SELECT * from T where T.A = @a and T.B IS NULL

For what its worth, this is in Microsoft SQL Server 2008

Answer 1

This should avoid the additional table access for the existence check. I'm not sure if there's a neater way.

SELECT * from T where T.A = @a and T.B = @b


IF (@@ROWCOUNT = 0)
BEGIN
    SELECT * from T where T.A = @a and T.B IS NULL
END