在这里看到了一个帖子 http://www.iteye.com/topic/1122917，很简单，凑下热闹，因为Oracle，SQLServer的子查询和Join算法的选择很多，但是MySQL方面就没有那么多的选择了，所以很多子查询的性能都很差，这里分析几个帖子里MySQL的答案。 需求：使用SQL语句查出哪些人即会玩basketball又会玩badminton，找出这样的name-hobby组合 create table test (NAME varchar(20) not null, HOBBY varchar(20) not null); insert into test values('Adam','basketball'); insert into test values('Bill','basketball'); insert into test values('Bill','football'); insert into test values('Cyper','basketball'); insert into test values('Cyper','badminton'); insert into test values('David','basketball'); insert into test values('David','badminton'); insert into test values('David','table tennis'); 一个Groupby的写法 select name,group_concat(hobby),count(*) from test where HOBBY IN ('basketball','badminton') group by name having count(*)]]]]>1; 一个exists的写法 select * from test a WHERE hobby = 'basketball' AND EXISTS (SELECT * from TEST b WHERE a.name = b.name AND b.hobby = 'badminton'); 但是有重复的情况如何处理？ insert into test values('Bill','basketball'); insert into test values('Bill','basketball'); insert into test values('David','badminton'); insert into test values('David','badminton'); Groupby的写法需要进化使用distinct select name,group_concat(distinct(hobby)), count(distinct(hobby)) from test where hobby in ('basketball','badminton') group by name having count(distinct(hobby))>1 exists的写法的写法不用修改，但是执行计划是DEPENDENT SUBQUERY，这样效率很差，那么要优化成Derived Table写法，如下 select distinct( a.name) as name,concat(a.hobby, b.hobby) from test a,(SELECT distinct(name) as name , hobby from TEST WHERE hobby = 'badminton') as b WHERE a.hobby = 'basketball' AND a.name = b.name --EOF--
转载于:https://www.cnblogs.com/buro79xxd/archive/2012/08/27/2658637.html
