tt540

SELECT client
FROM ( SELECT DISTINCT client
       FROM Dep
     UNION ALL
       SELECT client_id
       FROM Contact ) total
GROUP BY 1
HAVING COUNT(*) = 1

SELECT
    coalesce(contact.client_id, Dep.client) AS client_id
FROM 
    contact
    FULL JOIN Dep ON contact.client_id = Dep.client
WHERE
    contact.client_id is null
    or Dep.client is null
GROUP BY
    coalesce(contact.client_id, Dep.client)