SQL Server 2005 Query/trigger/function (whatever It Is That I Need)

Hey guys maybe you can help me out, been trying to figure this one out all day at work. I know how to use columns in a table to calculate another column in that same table. But I need to do some math on columns from a totally seperate table. Here is my scenario

 table 1 = stock table

 table 2 = Purchase order table

 in table 2 there are line items that have ordered quantities for parts that we have ordered

 in table 1 under each part number is a field for "quantity on order"

I need to compute the "quantity on order" in table 1 by summing all of the quantities in table 2 where the partnumber = the partnumber from table 1

 

quantity on order (table 1) = sum of all quantities (table 2) where the part numbers match

so for part number 516 i have this

 

table 2

poNumber             partNumber                 quantity

1                             516                             1

2                             516                             12

3                             516                             4

 

table 1

partNumber            inStock              onOrder

516                          0                        17(this is what i am trying to figure out how to compute)

 

any help on this qould be appreciated. I would like the database to automatically do this itself if at all possible.