MySQL - Cannot add or update a child row: a foreign key constraint fails

Question

This seems to be a common error, but for the life of me I can\'t figure this out.

I have a set of InnoDB user tables in MySQL that are tied together via foreign key;

Answer 1

I solved my 'foreign key constraint fails' issues by adding the following code to the start of the SQL code (this was for importing values to a table)

SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT;
SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS;
SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION;
SET NAMES utf8;
SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0;
SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0;
SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO';
SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0; 

Then adding this code to the end of the file

SET SQL_MODE=@OLD_SQL_MODE;
SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS;
SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS;
SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT;
SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS;
SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION;
SET SQL_NOTES=@OLD_SQL_NOTES; 

Answer 2

Hope this will assist anyone having the same error while importing CSV data into related tables. In my case the parent table was OK, but I got the error while importing data to the child table containing the foreign key. After temporarily removing the foregn key constraint on the child table, I managed to import the data and was suprised to find some of the values in the FK column having values of 0 (obviously this had been causing the error since the parent table did not have such values in its PK column). The cause was that, the data in my CSV column preceeding the FK column contained commas (which I was using as a field delimeter). Changing the delimeter for my CSV file solved the problem.

Answer 3

On an unrelated task, I recently brought up our MySQL database in MySQL Workbench, and when viewing the table relations for the above tables, I noticed 'duplicate' and/or spurious relations that I had somehow missed before (they weren't showing up in PHPMyAdmin FWIW). Removing these extra relations cleared up the issue immediately.

Answer 4

I've faced this issue and the solution was making sure that all the data from the child field are matching the parent field

for example, you want to add foreign key inside (attendance) table to the column (employeeName)

where the parent is (employees) table, (employeeName) column

all the data in attendance.employeeName must be matching employee.employeeName

Answer 5

I had the same problem but when I looked closely I found that, it was causing because I was trying to put the foreign key values into the tables before that key was assigned its primary key value. e.g. I had two tables "customers" and "films", "cust_id" and "film_id" were primary key respectively. "customer" had one-to-many relation with "films" so I had "cust_id" as foreign key in "films" tables. But I was trying to put values to "films" table first, so I got that problem.

Answer 6

Such an error on update may be caused by the difference in character set and collation so make sure they are the same for both tables.