During the preparation of my relational model, I'm stuck at the translation of a type constraint {XOR}.
Here's an example: I have a file that is equivalent credit by an applicant (a user) and it is also validated by a validator (another user) but there is an exclusion constraint the applicant may not be the validator to the same folder.
How I can translate it at my table?
Dossier_Equi (num_dosier, # applicant # validator ......)
First, note that it is not a type constraint (i.e. a constraint between two classifiers) but an association constraint. Then, note that the xor constraint specifies that objects of a class may participate in, at most, one of the associations at a time.
Constraint:
The applicant must not be the validator.
The SQL equivalent would be:
CHECK (validator IS NULL OR applicant<>validator)
(as a table constraint, since it applies to different columns).
However, MySQL does not support SQL check constraints. In another question, there is a suggerence for using BEFORE INSERT/UPDATE triggers for that purpose.
