Aggregate functions return one row for each group defined in a group by clause. Both column names and expressions can be used to define groups, and one group is a set of rows with the same values for all expressions specified in “group by.” Each row belongs to one and only one group. If “group by” is not specified, then the entire recordset is a single group, and in this case query always returns one row even if the recordset to be grouped is empty.
Listing 4-1 shows how to calculate the total amount of presentations and count of working days for all authors based on tables introduced in Listing ...
