How to Add same product name Qty in window application c#

Question

Inventory Table

ID	Date	Product name	Unit	Qty
1	10/10/2017	cello	pic	50
2	10/10/2017	notebook	pic	30
3	5/10/2017	Rice	kg	50
4	5/10/2017	Bottel	Litter	30
5	5/10/2017	notebook	pic	80
6	5/10/2017	cello	pic	40

Item in Stock

ID	Date	Product name	unit	totalQty	totalSold	StockInHand
1	10/10/2017	cello	pic	90	50	40
2	10/10/2017	notebook	pic	110	10	100
3	5/10/2017	Rice	kg	50	0	50
4	5/10/2017	Bottel	Litter	30	10	20

How to add two same name item like cello (50+40=90) and notebook (30+80=110)

sum(cast(inv.QTY as numeric(18,2)))+isnull(sum(cast(inv.QTY as numeric(18,2))),0) as totalQty