SELECT
 t.client_id AS `Client ID`,
 COUNT(t.id) AS `Total tickets`,
 CONCAT(cd.firstname,' ',cd.lastname) AS `Client`,
 cd.datecreated AS `Client since`
FROM hb_tickets t JOIN
 hb_client_details cd ON (cd.id=t.client_id)

GROUP BY t.client_id
ORDER BY `Total tickets` DESC
LIMIT 15