mysql foreign key error #1452

Question

ALTER TABLE  `groups` ADD FOREIGN KEY (  `company_id` ) REFERENCES  `summaries`.`companies` (

`id`
) ON DELETE CASCADE ;

MySQL said: 

#1452 - Cannot add or update a child row: a foreign key constraint fails (`summaries/#sql-164a_33c`, CONSTRAINT `#sql-164a_33c_ibfk_1` FOREIGN KEY (`company_id`) REFERENCES `companies` (`id`) ON DELETE CASCADE) 

companies.id is primary auto increment int(11)

company_id is index int(11)

I don't understand the error message. Can anyone shed some light on this?

Answer 1

That means you have at least one row in the child table that references a non-existent row in the parent table.

If you are absolutely sure that you are okay with having a data integrity issue like that, you can add the foreign key by disabling foreign key checks before you run the ALTER TABLE command:

SET FOREIGN_KEY_CHECKS = 0;