The FIRST FROM expression forms a record stream as specified
    by the record selection expression. If at least one record in
    the stream matches the record selection expression, Oracle Rdb
    uses the values stored in the first record of the record stream
    to evaluate the value expression. If there are no matches, you
    receive a runtime error.

    The FIRST FROM value expression can perform the equivalent of a
    table lookup when you are sure that the value you want to find is
    unique in a relation.