SELECT sort_label, release_id, track_info, sort_month, sort_title, sort_year, anv, sort_day, sub_type_id, is_main_release, artist_id, master_id, artist_credit_id, main_type_id,
IF(master_id, concat("M_",master_id), concat("R_", release_id)) AS bucket

FROM artist_credits_2 ac

WHERE artist_id = 27 AND main_type_id = "4" AND sub_type_id = "5"

GROUP BY bucket, main_type_id, sub_type_id

ORDER BY IF(sort_year>0, sort_year, 99999) ASC, IF(sort_month>0, sort_month, 99999) ASC, IF(sort_day>0, sort_day, 99999) ASC, sort_title ASC, is_main_release DESC


LIMIT 500