Question

I have a clients, feedback and a repairs tables. A client can give many feedbacks and have many repairs. In the feedback table I have created a clientid column (added also index) and I am able to create a foreign key to the clientid column (primary key) of the clients table.

The problem is that I am unable to do the same with the repairs table. Even though I have created a clientid column (indexed) within the repairs table and it has the same properties as the clientid within the clients table I get the following:

MySQL said: Documentation

#1452 – Cannot add or update a child row: a foreign key constraint
fails (ccsdb.,
CONSTRAINT #sql-3f0_8e5_ibfk_1 FOREIGN KEY (client_id) REFERENCES
clients (client_id) ON DELETE CASCADE ON UPDATE CASCADE)

Answer 1

That error (Cannot add or update a child row: a foreign key constraint fails) is referenced in the MySQL FK Doc

To add a reference between 2 tables the condition must fit for existing data.

That means if you say table1.id = table2.id then all ids of table1 and table2 must match together.

To solve that you have to eliminate or fix those rows that don’t match.
Example:

table1.id  |  table2.fk
   1       |       1      ok 
   2       |     null     error
   3       |       4      error if id 4 is not in table1