Datagridview sum of same identical rows

Question

I have a sql table basicly like this :

As you can see, there is data which uses same ProductID(1002 and 1004). My expectation of this table to become summed and merged with the rows of same ProductID.

I'm using c# and dataGridView. Can we sum those values via sql query then load it onto dataGridView ? or I must create a if - for etc.. statements to check & sum dataGridView rows ?

Edit :

For David's answer :

   select * from tblOrderDetail where OrderID in 
   (select OrderID from tblOrders whereCustomerID = 231234)

 // Select * at start gets the ProductID and Quantity 

How I can add your solution to this ? when I tried couple things I had errors about using "in" Clause.

Answer 1

SELECT   productID, SUM(Quantity) AS Quantity
FROM     myTable
GROUP BY productID