解决MySQL报错:You can‘t specify target table ‘region‘ for update in FROM clause
目录
- 前言
- 示例一
- 示例二
- 示例三
- 示例四
- 需要注意的地方
- 总结
前言
首先明确一点这个错误只会发生在delete
语句或者update
语句,拿update来举例 : update A表 set A列 = (select B列 from A表);
这种写法就会报这个错误,原因:你又要修改A表,然后又要从A表查数据,而且还是同层级。Mysql就会认为是语法错误!
嵌套一层就可以解决,update A表 set A列 = (select a.B列 from (select * from A表) a);
当然这个只是个示例,这个示例也存在一定的问题,比如(select a.B列 from (select * from A表) a)
他会查出来多条,然后赋值的时候会报 1242 - Subquery returns more than 1 row
。
嵌套一层他就可以和update撇清关系,会优先查括号里面的内容,查询结果出来过后会给存起来,类似临时表,可能有的人该好奇了,update A表 set A列 = (select B列 from A表);
我明明加括号了呀,难道不算嵌套吗,当然不算,那个括号根本没有解决他们之间的层次关系!
所谓层次关系就是一条sql当中谁先执行谁后执行,能理解层次关系的尽量要理解,不懂也没关系,下面我提供了四个案例 供大家参考!
示例一
以这张表为例:
DROP TABLE IF EXISTS `region`;CREATE TABLE `region`( `Id` int(11) NOT NULL COMMENT "主键id", `Name` varchar(40) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT "名称", `Pid` int(11) NULL DEFAULT NULL COMMENT "父类id", `status` varchar(2) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT "1" COMMENT "1:启用,2:禁用", PRIMARY KEY (`Id`) USING BTREE, INDEX `FK_CHINA_REFERENCE_CHINA`(`Pid`) USING BTREE) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = "省市区表" ROW_FORMAT = DYNAMIC;
错误用法:
这个例子就是典型,我要修改
region
表数据,要将pid
赋值为region
当中的某一列数据,这种写法就会报错!mysql不支持同一张表当中 既要修改又要查询
。
UPDATE region SET pid = (select pid FROM region WHERE NAME = "市辖区") where name="北京";
明确一点,这种的不是同一张表是不会报错的!
UPDATE region SET pid = (select id FROM banner) ;
正确用法:
UPDATE region SET pid = ( SELECT a.pid FROM ( SELECT Pid FROM region WHERE NAME = "市辖区" ) a ) WHERE NAME = "北京";
示例二
错误用法:
UPDATE region a SET Name = "1" WHERE a.pid IN ( SELECT id FROM region WHERE NAME = "市辖区" );
正确用法:
UPDATE region a SET Name = "1" WHERE a.pid IN ( SELECT b.id FROM (select * FROM region) b WHERE b.NAME = "市辖区" );
示例三
这个sql没有错误示例,只有正确示例,相对来说比较复杂点!
UPDATE region a SET pid = ( SELECT b.pid FROM ( SELECT id, pid FROM region b WHERE b.NAME = "市辖区" ) b WHERE a.Pid = b.id ) WHERE a.pid IN ( SELECT c.id FROM ( SELECT * FROM region ) c WHERE c.NAME = "市辖区" );
示例四
再来看一个删除的示例
错误用法:
delete from tbl where id in ( select max(id) from tbl a where EXISTS (select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1 ) group by tac);
正确用法:
delete from tbl where id in ( select a.id from (select max(id) id from tbl a where EXISTS( select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1)group by tac ) a);
需要注意的地方
(select...)
一定要加个别名,例如:(select...) a
否则报错如下:
总结
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