＃1452-无法添加或更新子行：外键约束失败

Question

I am trying to add Foreign Key on table "menu" and getting Error as:

1452 - Cannot add or update a child row: a foreign key constraint fails (`#sql-c74_c5`, CONSTRAINT `#sql-c74_c5_ibfk_1` FOREIGN KEY (`restaurant_id`) REFERENCES `restaurants` (`restaurant_id`)) 

My Restaurant table has one of the columns restaurant_id and it is Primary Key for this table.

Menu table has three columns menu_id , menu_name and restaurant_id . Now when i run

ALTER TABLE `menu` ADD FOREIGN KEY ( `restaurant_id` ) REFERENCES `restaurants` (`restaurant_id`);

then i got error #1452.

Please help me out of it :)

Answer 1

It sounds like the data already in that column does not meet the foreign key requirements, meaning, you either have a null value in restaurant_id in the "menu" table, or you have a value that does not exist in restaurant_id in restaurants. If you want the foreign key to only be enforced from this point on, you need a "nocheck", otherwise you need to have the data meet the requirements of the constraint. To add a nocheck foreign key, this is what you need:

ALTER TABLE menu with NOCHECK
ADD CONSTRAINT [FK_menu_restaurants] FOREIGN KEY ([restaurant_id])
REFERENCES restaurants([restaurant_id])

I hope this helps.