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

this worked for me :)

select * from Common

where

common_id not in (select ISNULL(common_id,'dummy-data') from Table1)

and common_id not in (select ISNULL(common_id,'dummy-data') from Table2)