How do I lookup the number of unique values from a list?

[Richard]

Hi,

I want to lookup the number of purchase orders placed by a particular vendor
from one sheet onto another. The problem I have is that the sheet that
contains the information shows purchase order lines which include the item
number, item description, qty purchased, the vendor name, and the purchase
order number for each line item on the PO. Is there a way to lookup a
particular vendor name, then count the number of unique purchase order
numbers? Your help would greatly be appreciated.


Thanks,