Problem adding Foreign Key using Alter Table with existing MYSQL Database - can't add it! Help!

Question

I have a production database where I have renamed several column\'s that are foreign keys. Obviously mysql makes this a real pain to do in my experience.

My solution

Answer 1

@egervari You wrote - My solution was to drop all the indexes and foreign keys, rename the id columns, and then re-add the indexes and foreign keys.

Agree with you. But it might be that something went wrong. I reproduced the error, and (in my case) fixed it.

I'd suggest you to run OPTIMIZE TABLE command for table where column was renamed. Documentation says - For InnoDB tables, OPTIMIZE TABLE is mapped to ALTER TABLE, which rebuilds the table to update index statistics and free unused space in the clustered index.

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One more solution:

Drop unique key in the referenced table (key that is used by foreign key, in your case it is a primary key). Then add new foreign key and recreate droped unique key.

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One more solution:

Try to add and drop new column to the referenced table, then try to create your foreign key.

ALTER TABLE company ADD COLUMN column1 VARCHAR(255) DEFAULT NULL;
ALTER TABLE company DROP COLUMN column1;