Prova detta (underfråga):
SELECT * FROM terms WHERE id IN
(SELECT term_id FROM terms_relation WHERE taxonomy = "categ")
Eller så kan du prova detta (GÅ MED):
SELECT t.* FROM terms AS t
INNER JOIN terms_relation AS tr
ON t.id = tr.term_id AND tr.taxonomy = "categ"
Om du vill ta emot alla fält från två tabeller:
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"