I wish to implement a type of xor constraint on foreign secrets in mysql 5.1

There's this table, let us say Entity which could make reference to two different types of valuesets symbolized by Tables ValsA and ValsB. Now I must implement a constraint wich makes certain that exactly among individuals two is planned, and also the other one is not.

In Oracle you could utilize something similar to

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

but so far as I realize it MySQL doesn't really support CHECK Constraints (yet).

Any ideas?

Correct. MySQL doesn't support check contraints. The CHECK clause is parsed but overlooked by all storage engines.

You'd need to enforce the XOR condition around the client side.