mysql查询各种类型的前N条记录，将3改为N（需查询条数）即可
(select * from event_info where event_type = 1 limit 3)
union all
(select * from event_info where event_type = 2 limit 3)
union all
(select * from event_info where event_type = 3 limit 3)
原文出处：http://my.oschina.net/u/1032146/blog/149300
