Jag kan inte testa det just nu men från vad jag hittade kan följande fungera:
Referens:Radnummervariabel
SQL-uttalande
SELECT @rownum := @rownum + 1 rownum
, t.*
FROM (
(SELECT t.id
, t.name
, c.company AS owner
, t.creation_date AS date
, t.notes
FROM tool t
, client c
WHERE t.id_customer = '15'
AND t.trash_flag = '1'
) UNION (
SELECT f.id
, f.name
, CONCAT(m.first_name, ' ', m.last_name) AS owner
, f.date
, f.notes
FROM file f
, meta m
WHERE ((f.acl = 0) OR (f.acl = 1 AND '1' = TRUE) OR (f.acl = 2 AND f.id = '7')) AND f.id = '15' AND f.trash_flag = '1' AND m.user_id = f.id_user)
)
) t
, (SELECT @rownum := 0) r
ORDER BY
'name' ASC
LIMIT 0, 20