Having clause i used to filter the results produced by GROUP BY clause. WHERE clause filters the result before selection while HAVING clause is used to filter the results produced by GROUP BY clause. That is the reason that WHERE clause is always used before GROUP BY clause and HAVING clause is used after GROUP BY clause. Here is an example to use HAVING clause for filtration of GROUP BY results.
SELECT OrderDate,SUM(TotalDue) TotalDueByOrderDate
WHERE OrderDate BETWEEN '7/1/2001' AND '7/31/2001'
GROUP BY OrderDate
Having Sum(TotalDue) > 7500
In result you may notice that HAVING clause filtered on result produced through GROUP BY clause and results qualified the condition in HAVING clause.