Calculates a value based on a value expression for every record
    in a record stream. When you use a statistical expression (except
    for COUNT), you specify a value expression and a record selection
    expression (RSE). Oracle Rdb first evaluates the value expression
    for each record in the record stream formed by the RSE. Then it
    calculates a single value based on all the results of the first
    step.  Statistical expressions are also called aggregate
    expressions.

    For more information on value expressions and record selection
    expressions, ask for HELP on Value_expr and RSE.