How To Use Cursor To Create Re-occuring Numbers?

SELECT pid, lname, visit_date, quantity
FROM customer
ORDER BY pid

pid lname visit_date quantity
-------------------------------------------
23 wang 07/23/2006 100
23 wang 07/30/2006 140
23 wang 08/05/2006 130
23 wang 08/15/2006 135
23 wang 08/22/2006 110
34 linden 06/23/2006 99
34 linden 07/06/2006 110
34 linden 07/15/2006 120
34 linden 08/26/2006 99

How do I use cursor (or is there a better way) to create an additional column "index"? "Index" starts at 1 for every unique pid


index pid lname visit_date quantity
----------------------------------------------------------
1 23 wang 07/23/2006 100
2 23 wang 07/30/2006 140
3 23 wang 08/05/2006 130
4 23 wang 08/15/2006 135
5 23 wang 08/22/2006 110
1 34 linden 06/23/2006 99
2 34 linden 07/06/2006 110
3 34 linden 07/15/2006 120
4 34 linden 08/26/2006 99

Please let me know if I can explain the my question better. Thanks in advance