The name of a relation or field constraint associated with the
    relation that is being defined. This name must be unique within
    the database. The constraint name can be referred to in other
    statements such as CHANGE RELATION, SHOW CONSTRAINT, and START_
    TRANSACTION.

    The clause 'CONSTRAINT constraint-name IS' is optional. If you
    do not specify the keyword CONSTRAINT, Oracle Rdb provides a name
    for the constraint. However, Rdb recommends that you always name
    field and relation constraints.  The alternative is to have
    constraints named by the database system with names such as
    LAST_NAME_REQUIRE_0001.