MySQL的去重复优化 在说去重复优化之前，我必须对fetch和duration这两个名词进行解读一下 mysql workbench中fetch和duration分别是什么意思 fetch： 它是指SQL在网络传输的过程中所用的时间，但不是SQL执行的时间 duration: 它指的是SQL语句执行的时间 下面有三种不同的SQL语句，最终的查询结果是一样的，但是SQL的执行时间不同 第一种：
select r.username,r.userid,r.photo from user r, (select t.userid from electricdevice natural join lessonuse u natural join lessonform right join tickup t on t.lessonuseid=u.lessonuseid where edeviceip='192.168.10.154' and date_sub(timestart,interval '00:15:00' hour_second)<=Now() and timeend>=Now() and date_add(timestart,interval '00:15:00' hour_second) >= tickuptime and date_sub(timestart,interval '00:15:00' hour_second) <= tickuptime) tt where r.userid=tt.userid;查询结果如下： 第二种：
select distinct(r.userid),r.username,r.photo from user r, (select t.userid from electricdevice natural join lessonuse u natural join lessonform right join tickup t on t.lessonuseid=u.lessonuseid where edeviceip='192.168.10.154' and date_sub(timestart,interval '00:15:00' hour_second)<=Now() and timeend>=Now() and date_add(timestart,interval '00:15:00' hour_second) >= tickuptime and date_sub(timestart,interval '00:15:00' hour_second) <= tickuptime) tt where r.userid=tt.userid;结果如下： 第三种：
select r.userid,r.username,r.photo from user r, (select distinct(t.userid) from electricdevice natural join lessonuse u natural join lessonform right join tickup t on t.lessonuseid=u.lessonuseid where edeviceip='192.168.10.154' and date_sub(timestart,interval '00:15:00' hour_second)<=Now() and timeend>=Now() and date_add(timestart,interval '00:15:00' hour_second) >= tickuptime and date_sub(timestart,interval '00:15:00' hour_second) <= tickuptime) tt where r.userid=tt.userid;总结： 在MySQL的SQL查询的过程中distinct去重的时候，如果能够在子查询中直接过滤的话，那么在子查询中过滤，因为这种情况下SQL的执行效率最高。
