Mysql 如何更改外键引用动作 (行为)

发布于01月17日

我设置了一个表，其中包含一个外键为ON DELETE CASCADE的列(删除父项时删除子项)

SQL命令是什么来将其更改为ON DELETE RESTRICT？(如果父项有子项，则无法删除它)

推荐答案

_{旧问题但添加答案以便获得帮助}

Its two step process:

假设一个table1有一个foreign key，列名为fk_table2_id，列名为constraint，列名为fk_name，table2是用键t2(something like below in my diagram)引用的表.

   table1 [ fk_table2_id ] --> table2 [t2]

First step，删除旧约束:(reference)

ALTER TABLE `table1` 
DROP FOREIGN KEY `fk_name`;

_{notice constraint is deleted, column is not deleted}

Second step，添加新约束:

ALTER TABLE `table1`  
ADD CONSTRAINT `fk_name` 
    FOREIGN KEY (`fk_table2_id`) REFERENCES `table2` (`t2`) ON DELETE CASCADE;

_{adding constraint, column is already there}

Example:

我有一张UserDetails号桌指的是Users号桌:

mysql> SHOW CREATE TABLE UserDetails;
:
:
 `User_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`Detail_id`),
  KEY `FK_User_id` (`User_id`),
  CONSTRAINT `FK_User_id` FOREIGN KEY (`User_id`) REFERENCES `Users` (`User_id`)
:
:

First step:

mysql> ALTER TABLE `UserDetails` DROP FOREIGN KEY `FK_User_id`;
Query OK, 1 row affected (0.07 sec)

Second step:

mysql> ALTER TABLE `UserDetails` ADD CONSTRAINT `FK_User_id` 
    -> FOREIGN KEY (`User_id`) REFERENCES `Users` (`User_id`) ON DELETE CASCADE;
Query OK, 1 row affected (0.02 sec)

结果:

mysql> SHOW CREATE TABLE UserDetails;
:
:
`User_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`Detail_id`),
  KEY `FK_User_id` (`User_id`),
  CONSTRAINT `FK_User_id` FOREIGN KEY (`User_id`) REFERENCES 
                                       `Users` (`User_id`) ON DELETE CASCADE
: