Is it not possible to have a check constraint reference a column that also has a foreign key?

Question

I'm on MySQL 8 trying to add a check constraint:

ALTER TABLE `table` ADD CHECK (
    (`column_a` IS NULL AND `column_b` IS NOT NULL) OR
    (`column_a` IS NOT NULL AND `column_b` IS NULL)
);

but I keep getting this error :

Column 'column_b' cannot be used in a check constraint 'table_chk_1': needed in a foreign key constraint 'table_ibfk_2' referential action.

I can't find any reference to this error anywhere else on the internet and I don't understand what the problem is. Both column_a and column_b are also foreign keys to other tables and they are both nullable. I just want to make sure that each row in table has either a reference via column_a or via column_b .

What is the cause of this error?

---

What have I tried

I've tried to drop the foreign keys, add the check constraints and it succeeds. Then if I add the foreign key back to column_b I still get the same error.

Answer 1

This is a documented behavior :

Foreign key referential actions ( ON UPDATE , ON DELETE ) are prohibited on columns used in CHECK constraints. Likewise, CHECK constraints are prohibited on columns used in foreign key referential actions.

So you need to choose between having a referential action on your column, or having a check constraint. Alternatively, you can keep the referential action and implement the check logic using triggers (or keep the check constraint and implement the referential action in a trigger.).