XOR Constraint in Mysql

I want to implement a kind of xor constraint on foreign keys in mysql 5.1

There is this table, let's say Entity which can refer to two different kinds of valuesets represente开发者_如何学Pythond by Tables ValsA and ValsB. Now I would like to implement a constraint wich makes sure that exactly one of those two is mapped, and the other one isn't.

In Oracle you could use something like

CHECK (NVL2(FK_A,1,0)+NVL2(FK_B,1,0)=1));

but as far as I understand it MySQL does not really support CHECK Constraints (yet).

Any ideas?

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Correct. MySQL does not support check contraints. The CHECK clause is parsed but ignored by all storage engines.

You'd have to enforce the XOR condition on the client side.