How to change the foreign key referential action? (behavior)

Question

I have set up a table that contains a column with a foreign key, set to ON DELETE CASCADE (delete child when parent is deleted)

What would the SQL com

Answer 1

_{Old question but adding answer so that one can get help}

Its two step process:

Suppose, a table1 has a foreign key with column name fk_table2_id, with constraint name fk_name and table2 is referred table with key t2 (something like below in my diagram).

   table1 [ fk_table2_id ] --> table2 [t2]

First step, DROP old CONSTRAINT: (reference)

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

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

Second step, ADD new CONSTRAINT:

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:

I have a UserDetails table refers to Users table:

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)  

result:

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
:

Answer 2

You can simply use one query to rule them all: 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

Answer 3

You can do this in one query if you're willing to change its name:

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

This is useful to minimize downtime if you have a large table.

Answer 4

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

Answer 5

Remember that MySQL keeps a simple index on a column after deleting foreign key. So, if you need to change 'references' column you should do it in 3 steps

• drop original FK
• drop an index (names as previous fk, using drop index clause)
• create new FK

Answer 6

I had a bunch of FKs to alter, so I wrote something to make the statements for me. Figured I'd share:

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'