Použijte:
SELECT t.*,
x.combinedsolutions
FROM TICKETS t
LEFT JOIN (SELECT s.ticket_id,
GROUP_CONCAT(s.soution) AS combinedsolutions
FROM SOLUTIONS s
GROUP BY s.ticket_id) x ON x.ticket_id = t.ticket_id
Náhradník:
SELECT t.*,
(SELECT GROUP_CONCAT(s.soution)
FROM SOLUTIONS s
WHERE s.ticket_id = t.ticket_id) AS combinedsolutions
FROM TICKETS t