you can't specify target table '' for update in from clause

ERROR 1093 (HY000): You can't specify target table 'A' for update in FROM clause 对这个错误mysql提供的信息是
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

遇到这种情况是可以这样
比如
mysql> update test1 as t1 ,test2 as t2 
        -> set t1.street=t2.address
        -> where t1.id=t2.test_id and t1.id in(select id from test1);
ERROR 1093 (HY000): You can't specify target table 't1' for update in FROM clause

mysql> create table t3 as select id as id from test1;
Query OK, 3 rows affected (0.03 sec)

mysql> update test1 as t1 ,test2 as t2 
        -> set t1.street=t2.address 
        ->where t1.id=t2.test_id and t1.id in(select id from t3);
mysql中修改字段的长度是利用modify,change 
alter table tbl_name modify 【column】 col_name column_type(长度)；

或者为：

alter table  tbl_name change 【column】 col_name  col_name varchar(41);

如果仅仅只是修改字段的类型或长度：

可以直接使用modify

如果想要同时改变字段名称和类型

使用change

 

mysql 去重可以使用distinct關鍵字 select  distinct name from testUser;