Hello everyone,

I want to share a nice, but undocumented, feature in Firebird (and
Interbase). Somehow, I am not sure whether or not I should use it.
Therefore, I am looking for some expert feed-back on its status (bug,
feature, will be documented, will disappear in time).

Suppose I want to extend the employee database to allow raising the
salary, but reject salary reductions. Such "transition" contraints are
usually programmed in TRIGGER code. But look at this CHECK constraint:

/* This works!! Let's consider it a feature. */
ALTER TABLE employee
ADD CONSTRAINT employee_salary_raise_ck
CHECK ((old.salary IS NULL) OR (new.salary >= old.salary));

Obviously, the nice thing is that "transition" constraints can be
defined declaratively.

So, what is the experts opinion?

Cheers,
Marco