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 command be to change this to ON DELETE RESTRICT
? (can't delete parent if it has children)
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
: