Output of sql query with group by clause

Question

A	B	C
5	2	1
10	NULL	2
15	2	3

In the above relation T the output of query:

select * from T group by B;

A	B	C
10	NULL	2
5	2	1

Can anyone explain why this is the output?

Answer 1

The group by command just return each row for unique combination of group by attributes. And every null is calculated as a unique group . i.e. if u have one more row then with NULL u will gate 3 rows. because the both the null will be calculated as a different group.
and it is not always the case it will return the first row. i think it is dependent on the database software . like on mysql i got first row, while on the online sql ide . http://www.w3schools.com/sql/trysql.asp?filename=trysql_select_all
this will produce the last row,

Answer 2

Remember one thing when you do group by clause then it will select the first row of each group so you are getting this answer