# 1、一张表中有 username 字段数据重复，查询出 username 字段数据重复的所有数据，如下
select * from table a where (a.username) in (select username from table group by username having count(*) > 1) order by username desc |
# 2、删除表中多余的重复记录，如下是删除企业信息表里企业统一信用代码（ unifiedcode ）重复的数据，通过 province_code 值不为空来判断应该删除的数据
delete from unit_company where id in (select A.id from (select id from unit_company D where D.unifiedcode in (select unifiedcode from unit_company group by unifiedcode having count(unifiedcode) > 1) and ifnull(D.province_code, '') = '') A ) |
# 3、查找表中多个字段重复的记录（多个字段），如下是查询 id 和 user_name 字段数据重复的所有数据
select * from table a | |
where (a.id,a.user_name) in (select id,user_name from table group by id having count(*) > 1) order by user_name desc |
