The name of a 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.