Question

I have a built database with all tables set using INNODB and Foreign Keys. Now I need to modify a single table and add a new column which references another table. My query is:

ALTER TABLE test ADD newcol INT NOT NULL;

ALTER TABLE test ADD CONSTRAINT fk_test
FOREIGN KEY (newcol) 
REFERENCES othertable(id);

I get the following error:

"#1452 - Cannot add or update a child row: a foreign key constraint fails."

All the other tables contain data, but I know that if I drop the test table and create it with foreign key it will work. If I drop test table it will delete a lot of records and I want to avoid copying data and re-inserting it.

Can anyone show me how to add a new column via a foreign key?

Answer 1

That error message means you have values in newcol that do not exist in othertable id column. By adding a column of INT NOT NULL, you set all the initial values for that column to 0. If you do not have an id of 0 in othertable, you have a key mismatch

Try this query:

SELECT  newcol 
FROM    test
WHERE   newcol NOT IN (
    SELECT  id
    FROM    othertable
)

if you get any results returned, then you have key values in newcol that violate the FK restraints