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