Check constraints are just SQL expressions, so in pesudocode it would be something like: (SELECT COUNT(*) shape JOIN square JOIN circle) = 1 How convenien

Check constraints are just SQL expressions, so in pesudocode it would be something like: (SELECT COUNT(*) shape JOIN square JOIN circle) = 1 How conveniently this can be expressed depend on the database system in question. But if we are talking about adding features to SQL, it could have built-in support for such a constraint, just as there is built-in support for say foreign key constraints.