explain select * from IMUser as u,(select *from IMMessage order by IMMessage.sendData desc) as m where (m.messageSenderId = u.openfireAccountId or m.messageReceiverId = u.openfireAccountId) and (u.openfireAccountId != 1234) group by u.openfireAccountId
原文：http://www.cnblogs.com/guligei/p/3607916.html