Essayez ceci (sous-requête) :
SELECT * FROM terms WHERE id IN
(SELECT term_id FROM terms_relation WHERE taxonomy = "categ")
Ou vous pouvez essayer ceci (JOIN):
SELECT t.* FROM terms AS t
INNER JOIN terms_relation AS tr
ON t.id = tr.term_id AND tr.taxonomy = "categ"
Si vous souhaitez recevoir tous les champs de deux tables :
SELECT t.id, t.name, t.slug, tr.description, tr.created_at, tr.updated_at
FROM terms AS t
INNER JOIN terms_relation AS tr
ON t.id = tr.term_id AND tr.taxonomy = "categ"