Vælg den maksimale værdi fra to tabeller

Opsummer foreningen af ​​to resuméforespørgsler, noget i stil med dette.

SELECT user_id, 
       MAX(user_date) user_date
  FROM
   (
     SELECT user_id, 
            MAX(last_contact) user_date 
       FROM emails_sent
      GROUP BY user_id
   UNION ALL
     SELECT whatever_user_id_column user_id, 
            MAX(whatever_date_column) user_date 
       FROM whatever_table
      GROUP BY user_id
   )a
GROUP BY user_id