一、需求，删除dname重复的记录，并且保留，id最小的，或最大的一行
id dname
1 事业部
2 销售部
3 技术部
4 测试部
5 技术部
6 事业部
7 事业部
1.按dname分组，统计条数
select
count(id) count,dname
from dept
group by dname
count dname
3 事业部
2 技术部
1 测试部
1 销售部
2.按dname分组，统计条数，筛选出count大于1的dname
select
dname
from dept
group by dname
having count(id)>1
dname
事业部
技术部
3.查询出所有有重复的记录
select * from dept
where dname in
(
select dname from dept
group by dname
having count(id)>1
)
4.按dname分组,取每组中id最小的记录
select min(id),dname from dept
group by dname
having count(id)>1
id
1
3
5.根据3的查询结果，除于4中的记录，就是要删除的记录
select * from dept
where dname in
(
select dname from dept
group by dname
having count(id)>1
)
and id not in
(
select min(id) id from dept
group by dname
having count(id)>1
)
id dname
5 技术部
6 事业部
7 事业部
6.删除指定的记录
delete from dept where id in(
select id from (
select * from dept
where dname in
(
select dname from dept
group by dname
having count(id)>1
)
and id not in
(
select min(id) id from dept
group by dname
having count(id)>1
)
)temptable
)