Dinesh Asanka hits on one of the under-utilized grouping operators:
You will see that data is aggregated for the columns provided by the GROUP BY clause. Important to note that the data will not be ordered in the GROUP BY columns and you need to explicitly order them by using the ORDER BY clause as shown in the above query.
In the above query, if you wish to find the total for Australia only, you need to run another GROUP BY with EnglishCountryRegionName and perform a UNION ALL. This will be a very ugly method. By using GROUP BY ROLLUP you can achieve the above-said task as shown in the following query.
If I were to rank grouping operators by how frequently I use them, it’s GROUPING SETS by a country mile, then ROLLUP, and almost never do I use CUBE.