Constraint-name must be unique within the database. The
    constraint name can be referred to in other statements such as
    DEFINE RELATION, SHOW CONSTRAINT, and START_TRANSACTION.

    The phrase 'CONSTRAINT constraint-name is' is optional. If you
    specify the keyword CONSTRAINT, you must also provide a name for
    the constraint.

    A field constraint definition differs from a relation constraint
    definition in that you cannot repeat referenced field names. See
    the "relation-constraint-def" help subtopic of CHANGE_RELATION
    for more information.