Normalize polymorphic foreign relationships using union all

polymorphic_join.sql

-- using union all to normalize

select id, name, foo, bar from (
  select * from messages
    left join broadcasts on broadcasts.id = messages.type_id
    where type = 'broadcast'

  union

  select * from messages
    left join auto_followups on auto_followups.id = messages.type_id
    where type = 'auto-follow-up'
) t;