mysql 删除重复数据，并保存最新一条数据

删除重复行

 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);

但是报1093错误

 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)    

 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

 DELETE FROM ecm_member_login_session
 WHERE (number , client_code) IN (
         SELECT number, client_code from (SELECT number, client_code FROM ecm_member_login_session GROUP BY number , client_code HAVING COUNT(*) > 1) A)
     AND update_time NOT IN (
         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：

 上面是目前MYSQL5.0仍然有的限制，文档中说：

 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:

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

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

 FROM

 clause. Example:

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

 Here the prohibition does not apply because a subquery in the

 FROM

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

 t

 have already been selected by the time the update to

 t

 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/