Question

How to get the information to update an already existing foreign key..

When updating a foreign key referemce in phpmyadmin this query is sent to the server

ALTER TABLE `dimension` ADD FOREIGN KEY (`test_id`) REFERENCES `db`.`test`
(`id`) ON DELETE CASCADE ON UPDATE RESTRICT;

But how do I get the status ON DELETE and ON UPDATE to sending and updating the alter table query?

Answer 1

You can’t. You have to drop the existing foreign key constraint and add a new one.

However, you can fetch the existing reference_option from the INFORMATION_SCHEMA:

SELECT CONSTRAINT_NAME, UNIQUE_CONSTRAINT_NAME, UPDATE_RULE, DELETE_RULE
FROM   INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS
WHERE  CONSTRAINT_SCHEMA = DATABASE()
   AND UNIQUE_CONSTRAINT_SCHEMA = DATABASE()
   AND TABLE_NAME = 'dimension'
   AND REFERENCED_TABLE_NAME = 'test'

Or else from SHOW CREATE TABLE:

SHOW CREATE TABLE dimension