Where clause operates on the selection before the rows are 
actually grouped. 

Hence , Try to add as many condition as possible in where 
clause instead of having to boost the SQL query performance.

where clause used to impose conditions in any dml operation
ex:
 selct * from emp where empid=10
having clause used to impose conditions on group data
ex:

select sum(sal),avg(sal),count(*) from emp 
having avg(sal)>2000

We can't use Where clause with aggregate function and Having
clause always use with aggregate function.

**Vinay Singh

having clause alwaye use with aggregate functions and where 
clause use for conditions but not with aggregate functions.