COUNT(forumre.main_id) AS num_re Max (If(forumre.re_time, forumre.re_time, forumre.main_time))
COUNT(forumre.main_id) AS num_re Max (If(forumre.re_time, forumre.re_time, forumre.main_time))
改为
COUNT(forumre.main_id) AS num_re,Max (If(forumre.re_time, forumre.re_time, forumre.main_time))
试试