I might be missing something here but after reading your SQL statement here is my 2 cents. Your LEFT join is being superceded byt the fact that you are essentially doing a straight JOIn by specifying WHERE thold.date = @asofdate and yhold.date = @prevdate. This WHERE clause makes it impossible for the LEFT join to grab record that don't match because your WHERE clause has told it to ignore all records unless there is a match on the date.