This clause names a field in the relation which is a part of
    a unique key. This field name can appear only once in the key
    definition.

    A UNIQUE clause or a PRIMARY KEY clause appearing at the relation
    level declares one or more fields to comprise a single unique or
    primary key.

    The UNIQUE clause limits field values such that no two rows in
    the associated relation can have the same non-missing values for
    the specified field or fields.