"Harry Jackson" <harryjackson@gmail.com> writes:
> If I made a mistake and changed the "id" column to "user_id" then from
> some the statement executes.
> =# delete from map_users_roles where user_id = (select user_id from
> users WHERE username = 'blah');
This is a standard "outer reference" construction, ie, user_id refers to
the field exposed in the outer-level query if there's no match in the
inner query. Possibly the SQL spec authors should have made it a little
harder to invoke an outer reference, but they didn't.
regards, tom lane