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
: