#删除emp_no重复的记录，只保留最小的id对应的记录。

1.先根据emo_no分组，找出每个的最小id，形成表t1。

select * from (

select min(id)

from titles_test

group by emp_no

) as t1

2.再用删除语句delete删除id不在t1表中的数据

delete from titles_test

where id not in t1

delete from titles_test
where id not in (
    select * from (
        select min(id)
        from titles_test
        group by emp_no
    ) as t1
)