--- having 

The `having clause' lets you select some of the groups formed by a
   previous `group by' clause and reject others, based on the
   results of an aggregate function.  This is the same as using an 
   aggregate function in a `where' clause which is not allowed.

If both a `where' and a `having' clause are present, the `where'
   clause is applied to select qualifying records, which are then
   grouped as indicated by the `group by' clause.  Finally, the
   qualifying groups are chosen by the `having' clause.

EXAMPLE:

   List the department number and average salary for departments
   having an average salary over 2000.

	  sql> select dept_no, avg(salary) from emp
	  sql> group by dept_no
	  sql> having avg(salary) > 2000/
