because instead of grouping one column you group a group of columns and make it a single column

for example

colA----------------------------colB
1-------------------------------1
1-------------------------------1
1-------------------------------2
2-------------------------------2
3-------------------------------1
3-------------------------------2
2-------------------------------2

if you use the query uday mentioned you are actually looking the distinct value

11
12
22
21
32

hm it´s a bit hard for me to explain, hope you understand it (this is like mathematics...)