Mysql error 1452 - Cannot add or update a child row: a foreign key constraint fails

Question

I\'m having a bit of a strange problem. I\'m trying to add a foreign key to one table that references another, but it is failing for some reason. With my limited knowledge o

Answer 1

This also happens when setting a foreign key to parent.id to child.column if the child.column has a value of 0 already and no parent.id value is 0

You would need to ensure that each child.column is NULL or has value that exists in parent.id

And now that I read the statement nos wrote, that's what he is validating.

Answer 2

Use NOT IN to find where constraints are constraining:

SELECT column FROM table WHERE column NOT IN 
(SELECT intended_foreign_key FROM another_table)

so, more specifically:

SELECT sourcecode_id FROM sourcecodes_tags WHERE sourcecode_id NOT IN 
(SELECT id FROM sourcecodes)

EDIT: IN and NOT IN operators are known to be much faster than the JOIN operators, as well as much easier to construct, and repeat.

Answer 3

I had the same problem today. I tested for four things, some of them already mentioned here:

1. Are there any values in your child column that don't exist in the parent column (besides NULL, if the child column is nullable)

2. Do child and parent columns have the same datatype?

3. Is there an index on the parent column you are referencing? MySQL seems to require this for performance reasons (http://dev.mysql.com/doc/refman/5.5/en/create-table-foreign-keys.html)

4. And this one solved it for me: Do both tables have identical collation?

I had one table in UTF-8 and the other in iso-something. That didn't work. After changing the iso-table to UTF-8 collation the constraints could be added without problems. In my case, phpMyAdmin didn't even show the child table in iso-encoding in the dropdown for creating the foreign key constraint.