判断数据库是否有某条记录：select 1 from xxx where xxx (效率：1> colname > *)
清空表记录: truncate table xxx > delete from xxx
多表查询： select * from xxx A left join xxx B on A.x = B.x where xxx > select * from xxx A where A.x in (select B.x from xxx B);
原文：http://www.cnblogs.com/itpoorman/p/4422490.html