Order by clause is used to sort information of a table into 
specific order either ascending or descending order.

syntax:select stmt order by column;

ex:select * from emp order by sal;

Group by clause is used to divide entire group into 
subgroups based on the common values columns.we can use 
group by clause along with group functions.

syntax:select group by(exp 1),group by(exp 2),....from 
table name group by column name;
select max(sal),min(sal) from emp group by deptno;
Group by clause can allow where clause.


Having:is used to restrict information of the atble based 
on condition.

syntax:select group by (exp 1) ,group by (exp 2) from table 
name group by column having (condition).

Having and Group by clauses can interchange their positions.

but when using having along with order by clause having 
should be placed before order by clause.

sory i m satisfied with idea.actualy i want to know is this
query is right or not.plz inform me.

SELECT country,city
FROM customers 
GROUP BY country
ORDER BY city DESC