无法添加或更新子行：外键约束失败。 这是什么？

Question

sorry, i am still new in php... i want to ask... how actually foreign key is functional? i mean, i have create a foreign key and primary key, everything fine. i try look in internet, but still i cant make it work or did mistake that i dont know. i get this error "Cannot add or update a child row: a foreign key constraint fails" when i try to insert information inside my other table that have foreign key

my parent table "user_information" have:

user_id = primary key
user_password
name
user_category
group_id

and my other "table vehicle_registration" have:

plate_number = primary key
user_id = foreign key
roadtax_validation
vehicle_category
user_option
insurance name

i want to make the user that log in can insert their vehicle registration, and it bind to that user, the user_id should be similar, just other information is diferent... how i can make it? is it correct that the foreign key is to work like that? cause when i try to insert value inside the vehicle_registration it will show "Cannot add or update a child row: a foreign key constraint fails" can someone explain how it actually done?

Answer 1

You have to add user/s first. You can't add a record in vehicle_registration table if you dont have a matching record in the parent table.

That is how foreign key constraint works. For example if you have a record in user_information table with user_id = 1, you are only allowed to have records with user_id = 1 in the vehicle_registration table. And so on...