If some one tell me where exactly I use 'having' and 'where' ...

Answer 1

having is used to select group and used in groupby clause

Answer 2

where eg: select name from emp(table name) where id in(1,3,6)
having is same as where but used after group by clause
eg: select * from emp(table name)select sum(salary),deptno from emp group by deptno
having sum(salary)>5000