SELECT mm.*, md.label AS standardized_label
FROM mytable md
LEFT JOIN
mytable mc
ON mc.parent_element = md.id
AND mc.client = @client
JOIN mytable mm
ON mm.id = COALESCE(mc.id, md.id)
WHERE md.client IS NULL
Създайте индекс на (client, parent_element)
за да работи бързо.
Вижте SQLFiddle .