A null in the result row of a grouped query containing CUBE, ROLLUP, or GROUPING SET can mean one of the following:
- The actual data for the column is null.
- The extended grouping specification aggregated over the column and excluded it from the particular grouping. A null in this case really represents all values for this column.
Use GROUPING to distinguish between rows with nulls in actual data from rows with nulls generated from grouping sets.