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

If you want the world to be a two-valued boolean place, you must prevent the null (third value) case yourself.

Don't write IN clauses that allow nulls in the list side. Filter them out!

common_id not in
(
  select common_id from Table1
  where common_id is not null
)