Question

I need to find all id’s from the OldTable that doesn’t exist in the NewTable
Why won’t this query find the id?

SELECT old_id FROM OldTable WHERE old_id NOT IN (
SELECT id FROM NewTable)  

By them selves they return this

--Returns the id 18571
SELECT old_id FROM OldTable WHERE old_id = 18571

 --Returns nothing  
SELECT id FROM NewTable WHERE id = 18571 

Am I missing something obvious here?

Both columns are of type int and primary keys.

SOLVED

The id column had null’s in them, I was just being ignorant =/

These works:

SELECT old_id FROM OldTable EXCEPT SELECT id FROM NewTable  

SELECT * FROM old_table ot WHERE NOT EXISTS (
SELECT * FROM new_table nt WHERE nt.id = ot.old_id)  

These doesn’t work:

SELECT old_id FROM OldTable LEFT JOIN NewTable ON old_id = id WHERE id IS NULL  

SELECT old_id FROM OldTable WHERE old_id NOT IN (
SELECT id FROM NewTable)

Answer 1

SELECT * FROM old_table ot
WHERE NOT EXISTS (
   SELECT * FROM new_table nt
   WHERE nt.new_key = ot.old_key
   );