SQL & PL/SQL :: Unique Values - Print Output Of Table

I am stuck with this query.

I have a table "xyz" as -->>

SQL> select * from xyz;

A B
---------- ----------------------------------------
1 Hello
2 Hello
3 No Hello
4 No Hello
5 Hello
6 Hello
7 Hello

I want to print the output of this table as -->>

A B
---------- ----------------------------------------
1
2 Hello
3
4 No Hello
5
6
7 Hello

To make it more clear, I just want that whenever the value f column "B" changes then only its value should be printed, else it should be NULL. And if "B" has same value for all the records then the value of "B" should be printed at the last.

Either of SQL or PLSQL would d for me.