转自：http://www.360doc.com/content/18/0625/16/833305_765289549.shtml

依下图Students表内容为例,Id_P列不同,Id_P类型为int,自增字段

一、查询重复的数据

（Name,Address,City列值重复）

SELECT Name, Address, City, COUNT(*) AS CountStudents

FROM Students

GROUP BY Name, Address, City

HAVING (COUNT(*) > 1)

查询结果如下图：

通过查询结果可以看出Name,Address,City列值重复的有哪些及重复次数

SELECT MAX(Id_P) AS MAXId_P

FROM Students

GROUP BY Name, Address, City

查询结果如下图：

依Name, Address, City分组查询Id_P最大值，这样查询的结果就不包含重复的Id_P值

SELECT *

FROM Students

WHERE (Id_P IN

(SELECT MAX(Id_P) AS MAXId_P

FROM Students AS Students_1

GROUP BY Name, Address, City))

查询结果如下图：

查询结果没有重复的数据，可以把MAX（Id_P）改成MIN（Id_P）

DELETE FROM Students

WHERE     (Id_P NOT IN

(SELECT MAX(Id_P) AS MAXId_P

FROM Students AS Students_1

GROUP BY Name, Address, City))

删除重复的数据，注意Id_P后面"NOT IN"