FIRST value-expr FROM rse

    Specifies a value by forming a record stream (as indicated by a
    record selection expression). If at least one record matches the
    RSE, the values stored in the first record of the record stream
    are used to evaluate the value expression.

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

    The value expression, value-expr, is a symbol or a string of
    symbols used to calculate a value. The rse parameter specifies a
    clause that products use at run time to include specific records
    for processing.