今天在做数据订正的时候，写了一个简单的子查询语句，想要把名字重复的应用删除掉一个，删掉其中id号比较大的那个，具体SQL如下：
delete from appinfo where id in (select max(id) from appinfo group by app_name having count(app_name )>1)
But，报错了。
失败, 详情: You can't specify target table 'appinfo' for update in FROM clause
原因：
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 ...);
所以，将SQL语句改成下面的内容，执行成功。
delete from appinfo where id in
(
select a.id from
(
select max(id) id from appinfo a group by app_name HAVING count(app_name)>1
) a
)
