题解 | #删除emp_no重复的记录，只保留最小的id对应的记录。#

思路：有两个方法可以解决问题

方法一：

1. 先通过对 emp_no 进行分组，找出最小的 id

select min(id) from titles_test group by emp_no

2. 使用 not in 删除其他id

delete from titles_test
where id not in 
(select * from (select min(id) from titles_test group by emp_no) as tmp);

方法二：

1. 我们首先找出所有出现频次 > 1 的 emp_no 及 id

select min(id) as id, emp_no
from titles_test
group by emp_no
having count(emp_no) > 1

2. 然后我们连接 titles_test 表，找出所有需要删除的 id

select a.id
from titles_test as a, (select min(id) as id, emp_no
from titles_test
group by emp_no 
having count(emp_no) > 1) as b
where a.emp_no = b.emp_no and a.id > b.id

3. 最后根据 id 删除数据

delete from titles_test
where id in (select * from (select a.id
from titles_test as a, (select min(id) as id, emp_no
from titles_test
group by emp_no 
having count(emp_no) > 1) as b
where a.emp_no = b.emp_no and a.id > b.id
) as tmp)