如何更改外键引用操作(行为)

_{老问题,但添加答案,以便可以得到帮助}

分两步进行:

假设,一个 table1 有一个 外键 具有列名 fk_table2_id 约束 名称 fk_name 和 table2 t2 ( ).

   table1 [ fk_table2_id ] --> table2 [t2]

,删除旧约束:( reference

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

_{注意约束被删除,列不被删除}

,添加新约束:

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

例子:

我有一个 UserDetails 表引用 Users 表格:

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`)
:
:

第一步:

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

第二步:

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)  

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
:

您可以在一个查询中完成此操作 如果 你愿意改变它的名字:

ALTER TABLE table_name
  DROP FOREIGN KEY `fk_name`,
  ADD CONSTRAINT `fk_name2` FOREIGN KEY (`remote_id`)
    REFERENCES `other_table` (`id`)
    ON DELETE CASCADE;

如果您有一个大表,这有助于最小化停机时间。

ALTER TABLE DROP FOREIGN KEY fk_name;
ALTER TABLE ADD FOREIGN KEY fk_name(fk_cols)
            REFERENCES tbl_name(pk_names) ON DELETE RESTRICT;

请记住,MySQL在删除外键后在列上保留一个简单的索引。因此,如果您需要更改“references”列,您应该分3个步骤来完成

• 删除原始FK
• 删除索引(名称为previous fk,使用 drop index 条款)
• 创建新FK

我有一堆FK要修改,所以我写了一些东西来为我做陈述。我想我会分享:

SELECT

CONCAT('ALTER TABLE `' ,rc.TABLE_NAME,
    '` DROP FOREIGN KEY `' ,rc.CONSTRAINT_NAME,'`;')
, CONCAT('ALTER TABLE `' ,rc.TABLE_NAME,
    '` ADD CONSTRAINT `' ,rc.CONSTRAINT_NAME ,'` FOREIGN KEY (`',kcu.COLUMN_NAME,
    '`) REFERENCES `',kcu.REFERENCED_TABLE_NAME,'` (`',kcu.REFERENCED_COLUMN_NAME,'`) ON DELETE CASCADE;')

FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS rc
LEFT OUTER JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcu
    ON kcu.TABLE_SCHEMA = rc.CONSTRAINT_SCHEMA
    AND kcu.CONSTRAINT_NAME = rc.CONSTRAINT_NAME
WHERE DELETE_RULE = 'NO ACTION'
AND rc.CONSTRAINT_SCHEMA = 'foo'

ALTER TABLE products DROP FOREIGN KEY oldConstraintName, ADD FOREIGN KEY (product_id, category_id) REFERENCES externalTableName (foreign_key_name, another_one_makes_composite_key) ON DELETE CASCADE ON UPDATE CASCADE