NULLs in subselects do bite me with distressing regularity: Writing SELECT ... FROM ... WHERE blah NOT IN (SELECT foo FROM bar); getting no hits until I sla

NULLs in subselects do bite me with distressing regularity: Writing SELECT ... FROM ... WHERE blah NOT IN (SELECT foo FROM bar); getting no hits until I slap my forehead and add WHERE foo IS NOT NULL to the subselect.