Mysql：无法添加或更新子行：外键约束失败

Question

First of all, i know there are many posts about this subject but the ones i checked didnt help me so i wanted to post my problem.

I am building a website that people can register as user then fill some different forms for me. I have 2 table one is "user" and the other one is "form" table. In user table i have user_id which is auto_incremented and this id connected to "form" table as foreign key. I want to see which user filled forms via user_id. When i try to register filled form to database i get "Cannot add or update a child row: a foreign key constraint fails" message. How can i fix this?

Answer 1

If you have set the user_id on on your form table as a foreign key pointing at the user_id in the user table, you must supply a valid user_id when you insert the form row.

You could set the user_id field on the form table to be allowed to be NULL this will mean it does not have to be set.

Answer 2

您正在尝试将一些数据添加到外键列中，而其他表中没有相同的数据。

Answer 3

This mostly happens when there is a data in table and you want to create/ edit/ delete a foreign_key
OR
the field that you want to make it foreign_key is not the same format with Table field

solution:
You have to make a
1)backup or copy of you data
2)remove the foreign_keys
3)and Truncate data from table
4)Finally re attach the foreign_keys
5)update your data

I hope it helps