关于mysql数据库中重复数据的操作

 1 SELECT
 2     *
 3 FROM
 4     A
 5 WHERE
 6     idcard IN (
 7         SELECT
 8             idcard
 9         FROM
10             A
11         GROUP BY
12             idcard
13         HAVING
14             count(idcard) > 1
15     )

 1 SELECT  *  FROM
 2 
 3 　　A a,B b
 4 
 5 WHERE
 6 
 7 　　a.idcard=b.idcard
 8 
 9 GROUP BY
10 
11 　　b.idcard

 1 <!-- 删除表中重复的数据 -->
 2 DELETE FROM A
 3 WHERE
 4     idcard IN (SELECT
 5         *
 6     FROM
 7         (SELECT
 8             idcard
 9         FROM
10             A
11         GROUP BY idcard
12         HAVING COUNT(idcard) > 1) t1)
13     AND id NOT IN (SELECT
14         *
15     FROM
16         (SELECT
17             MIN(id)
18         FROM
19             A
20         GROUP BY idcard
21         HAVING COUNT(idcard) > 1) t2)

1 <!-- 查找A表有B表没有的数据 -->
2 SELECT * 
3     from A a 
4 WHERE NOT EXISTS(
5     SELECT idcard 
6         from B b
7     WHERE
8         a.idcard=b.idcard
9 )

<!-- 也可以这样写 -->

SELECT

　　*

　FROM

　　A

　WHERE 

　　id NOT IN (

　　　　SELECT id FROM B

　　)