CHECK constraint not implemented in MySQL, what about other DBMS?
Every R (relational) DBMS I've used has.  MySQL is peculiar in so many ways I struggle to put it head to head with any of the "normal" RDBMS for anything other than a basic data dump for a simple application.

There are plenty of DBs that are not relational that don't have the concept.  As far as I know though, there are none other of either sort that support the syntax without the functionality.