怎么在mysql里查询重复的数据？查询筛选重复数据sql语句

怎么在mysql里查询重复的数据？查询筛选重复数据sql语句

不多说 请仔细看下面

查询重复数据数量

select device_id from device group by device_id having count(device_id) > 1; 查询所有重复数据

select userid device_id create_date from device where device_id in (select device_id from device group by device_id having count(device_id) > 1) order by device_id create_date desc ; 重复一条中create_date 最新的那一条

select max(create_date) from device group by device_id having count(device_id)>1; 筛选查询

select * from device where device_id in (select device_id from device group by device_id having count(device_id) > 1) and create_date not in (select max(create_date) from device group by device_id having count(device_id)>1) order by device_id create_date desc ; 下面再看一些实例吧

表结构如下：

MySQL> desc test1; -------------- ------------------ ------ ----- --------- ---------------- | Field | Type | Null | Key | Default | Extra | -------------- ------------------ ------ ----- --------- ---------------- | ID | int(10) unsigned | NO | PRI | NULL | auto_increment | | SENDERNAME | varchar(32) | YES | | NULL | | | RECEIVERNAME | varchar(64) | YES | | NULL | | | SENDER | varchar(64) | NO | | NULL | | | RECEIVER | varchar(64) | NO | | NULL | | | subject | varchar(512) | NO | | NULL | | | CONTENT | text | NO | | NULL | | | PRIORITY | int(11) | NO | MUL | NULL | | | STATUS | int(11) | NO | MUL | NULL | | | CREATETIME | datetime | NO | | NULL | | | SENDTIME | datetime | YES | | NULL | | -------------- ------------------ ------ ----- --------- ----------------

subject和RECEIVER 需要做uniq key 但设计时未做，后面的数据就有很多重复的记录。

1. 查询需要删除的记录，会保留一条记录。

select a.id a.subject a.RECEIVER from test1 a left join (select c.subject c.RECEIVER max(c.id) as bid from test1 c where status=0 GROUP BY RECEIVER SUBJECT having count(1) >1) b on a.id< b.bid where a.subject=b.subject and a.RECEIVER = b.RECEIVER and a.id < b.bid 2. 删除重复记录，只保留一条记录。注意，subject RECEIVER 要索引，否则会很慢的。

delete a from test1 a (select c.subject c.RECEIVER max(c.id) as bid from test1 c where status=0 GROUP BY RECEIVER SUBJECT having count(1) >1) b where a.subject=b.subject and a.RECEIVER = b.RECEIVER and a.id < b.bid;

好了筛选重复数据的sql语句大概就这些了，如果你能理解那几乎不担心重复数据这一说了。