row numbering

Hello list,

is there a way return a column with the row number automatically 
generated according the way the rows were processed by the query.

For instance:
select a,b from foo;
a  b
20 yes
40 no
15 yes

to something like:

select counter(),a,b from foo;
counter a  b
1	20 yes
2	40 no
3	15 yes

Thanks in advance,


-- 
Sinceramente,
Josué Maldonado.

... "El bien supone la luz, el mal tinieblas. Cada vez hay menos 
iluminación."