this is my table with columns:

 bus_quarter | bus_year
-------------+----------
           4 |     2006
           1 |     2007
           2 |     2007
           3 |     2007
           4 |     2007
           1 |     2008
           2 |     2008
           3 |     2008
           4 |     2008
           1 |     2009
           2 |     2009
           3 |     2009
           4 |     2009

now i want to add one column as id 
whose value shd be serial;
how can i do it using SQL?