CASE 

WHEN `old`.`date_deleted` IS NULL AND `new`.`date_deleted` IS NOT NULL
THEN UPDATE `forum_topic` SET `count_replies` = `count_replies`-1 WHERE `id` = `new`.`topic_id`;

WHEN `old`.`date_deleted` IS NOT NULL AND `new`.`date_deleted` IS NULL
THEN UPDATE `forum_topic` SET `count_replies` = `count_replies`+1 WHERE `id` = `new`.`topic_id`;

END CASE;