MySQL的去重复优化

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。