create table lovepair (boyfrndname varchar(10),girlfrndname 
varchar(10))

insert into lovepair values ('BF1','GF1')
insert into lovepair values ('BF1','GF2')
insert into lovepair values ('BF1','GF3')
insert into lovepair values ('BF2','GF1')
insert into lovepair values ('BF2','GF2')
insert into lovepair values ('BF3','GF3')
insert into lovepair values ('BF3','GF3')
insert into lovepair values ('BF3','GF3')

-- Here BF1 have 3 Girl Friends. like the BF2 and BF3 have 
2 and 3 Girl Friends Simultaneously.

-- Here is the query for, how many girls friends each boys 
have ???? USING group by

SELECT  boyfrndname, COUNT (*)
FROM	lovepair
GROUP BY boyfrndname

// Here another qry for who have more than 2 girl 
friends ?? USING GroupBy and Having .

// Having is used for applying some condition in Aggregate 
function

SELECT boyfrndname,COUNT(*)
FROM	lovepair
Group BY boyfrndname
having  count(*)>2


--- Now u clear...........

Group by is used To Group Result According To Group Name
Having Is Always Is Used With Group by Clause.
Having Contain Aggregate Function or Simple Sub Query .
as Show In Above Example