SELECT
d.id,
d.name,
CONCAT_WS(' -> ',c5.name,c4.name,c3.name,c2.name,c1.name) as cat_string
FROM domain d
LEFT JOIN category c1
ON c1.id = d.category_id
LEFT JOIN category c2
ON c1.parent_id = c2.id
LEFT JOIN category c3
ON c2.parent_id = c3.id
LEFT JOIN category c4
ON c3.parent_id = c4.id
LEFT JOIN category c5
ON c4.parent_id = c5.id;