mysql distinct 查询疑问

问题描述

需求

想知道最近N条记录中某一字段取值有哪几种

select * from t;+----+------+| id | a |+----+------+| 1 | aaa || 2 | aaa || 3 | bbb || 4 | bbb || 5 | ccc || 6 | ddd || 7 | ddd || 8 | foo || 9 | bar |+----+------+# 想知道最早4条记录中 a取值有哪几种 期望是aaa bbb 但实际不满足期望select distinct a from t order by id limit 4;+------+| a |+------+| aaa || bbb || ccc || ddd |+------+#必须使用这种写法select distinct a from (select a from t order by id limit 4) a;+------+| a |+------+| aaa || bbb |+------+

为什么第一种写法不行？ 似乎是先将所有a的取值都查出来再截取4个，但此时没有id啊，只有a啊。Mysql又是怎样处理order by id的呢？

问题解答

这是由于sql的执行顺序来决定的.写的顺序：select ... from... where.... group by... having... order by.. limit [offset,] (rows)执行顺序：from... where...group by... having.... select ... order by... limit可以出来,limit是最后一个被执行的.看你的sql,其实是先找出所有的distinct(a),然后再limit 4(4个distinct a) .

首先Explain一下

mysql> explain select * from t order by id limit 4;+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+| 1 | SIMPLE | t | index | NULL | PRIMARY | 4 | NULL | 4 | NULL |+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+

然后加入distinct

mysql> explain select distinct a from t order by id limit 4;+----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------+| 1 | SIMPLE | t | index | NULL | PRIMARY | 4 | NULL | 4 | Using temporary |+----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------+1 row in set (0.00 sec)

区别是Extra ： Using temporary，即distinct用临时表保存中间结果。

所以可以这样理解，执行结果是把select distinct a from t放到了临时表，然后再从临时表取出数据，做了where、 order by操作。