@PaulWhite You are just the perfect person to answer, wow, thanks! Take a look at
this SqlFiddle. Having doubts about whether the first query would truly short-circuit the second table lookup, I cooked up another query that theoretically could properly short-circuit. From what you've said, and looking at the execution plan for the second query, I believe that it may work as hoped: not taking extra work for any row where the value is found in the first table.