SQL “select where not in subquery” returns no results

Question

Disclaimer: I have figured out the problem (I think), but I wanted to add this issue to Stack Overflow since I couldn\'t (easily) find it anywhere. Also, someone might

Answer 1

SELECT T.common_id
  FROM Common T
       LEFT JOIN Table1 T1 ON T.common_id = T1.common_id
       LEFT JOIN Table2 T2 ON T.common_id = T2.common_id
 WHERE T1.common_id IS NULL
   AND T2.common_id IS NULL