mysql 删除重复数据，并保存最新一条数据

 

删除重复行

1 DELETE FROM ecm_member_login_session 

2 WHERE (number , client_code) IN (

3         SELECT number, client_code FROM mall.ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1) 

4     AND update_time NOT IN (

5         SELECT MAX(update_time) FROM mall.ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1);

 

但是报1093错误

1 14:57:04    DELETE FROM ecm_member_login_session  WHERE (number , client_code) IN (   SELECT number, client_code FROM mall.ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1)   AND update_time NOT IN (   SELECT MAX(update_time) FROM mall.ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1)    

2 

3 Error Code: 1093. You can't specify target table 'ecm_member_login_session' for update in FROM clause    0.046 sec

 

后来在 嵌套查询语句里面再嵌套一层即可， 是因为mysql限制update（包括delete）操作表名与嵌套子查询语句里的表名一样的原因。

 

语句改为如下

1 DELETE FROM ecm_member_login_session 

2 WHERE (number , client_code) IN (

3         SELECT number, client_code from (SELECT number, client_code FROM ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1) A) 

4     AND update_time NOT IN (

5         SELECT update_time from (SELECT MAX(update_time) FROM ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1) A);

 

 mysql文档：

 1 上面是目前MYSQL5.0仍然有的限制，文档中说：

 2 

 3 In general, you cannot modify a table and select from the same table in a subquery. For example, this limitation applies to statements of the following forms:

 4 

 5 DELETE FROM t WHERE ... (SELECT ... FROM t ...);UPDATE t ... WHERE col = (SELECT ... FROM t ...);{INSERT|REPLACE} INTO t (SELECT ... FROM t ...);

 6 

 7 Exception: The preceding prohibition does not apply if you are using a subquery for the modified table in the

 8 

 9 FROM

10 

11 clause. Example:

12 

13 UPDATE t ... WHERE col = (SELECT (SELECT ... FROM t...) AS _t ...);

14 

15 Here the prohibition does not apply because a subquery in the

16 

17 FROM

18 

19 clause is materialized as a temporary table, so the relevant rows in

20 

21 t

22 

23 have already been selected by the time the update to

24 

25 t

26 

27 takes place. 

 

 

 

 

 

参考：

mysql error 1093 解决方法

SQL删除重复数据只保留一条

 http://blog.163.com/xiaoqiu_1120/blog/static/121632322007112411424982/