Query to calculate Quantity on Hand

Question

i want to calculate the quantity when the product is sale that come in stock table with product_id and quantity add in stock_out

and when the purchase then quantity add in Stock_in can u plz tell me how to make the query to know the quantity 

 

 

SELECT TOP 1000 [id]

,[product_id]
,[stock_date]
,[stock_in]
,[stock_out]
,[vocher_type]
,[vocher_id]
FROM [OnlineTradingFinal].[dbo].[Stock]