How To Count Groups Returned with the GROUP BY Clause?

If you use the COUNT(*) function on groups returned with the GROUP BY clause, it will count the
number of rows within each group, not the number of groups. If you want to count the number of
groups, you can put the GROUP BY query into a subquery and apply the COUNT(*) function on
the main query as shown in the following tutorial exercise:
SQL> SELECT first_name, COUNT(*) FROM employees
GROUP BY first_name HAVING COUNT(*) > 1;
FIRST_NAME COUNT(*)
-------------------- ----------
Peter 3
Michael 2
Steven 2
John 3
Julia 2
William 2
Karen 2
Kevin 2
......
SQL> SELECT COUNT(*) FROM (
SELECT first_name, COUNT(*) FROM employees
GROUP BY first_name HAVING COUNT(*) > 1
);
COUNT(*)
----------
13