With Rollup

The 'RollUp' Sub-Clause

SELECT Age, Name, COUNT(Name)
FROM Person 
GROUP BY Age WITH ROLLUP
HAVING Age > 50

Adds an additional record item of the results of the initial aggregation option.

So: if you had a grouping of COUNT()'s for each Age already totaled up, the ROLLUP looks at all of those totals and aggregates them into a grand total.