How to hide the zero value of SQL join query.

I don't want to see a stock of products that are empty.
 I can't figure out my mistake " where condition".
Thanks in advance for the solution.

select a.Company_Name as Company, 
           a.Product_name as product,
          stock =ISNULL( a.qty,0) - isnull(b.qty, 0)-isnull(c.Qty,0)
    from   (
            select Company_Name, Product_name, qty = sum(Qty)
            from   tbl_purchase
             group by Company_Name, Product_name
           ) a
           left join 
           (select Company_Name, Product_name,qty = sum(qty)
             from  tbl_sales
    		   group by Company_Name, Product_name
           ) b     on  a.Company_Name = b.Company_name
                 and a.Product_name = b.Product_name
   
	left join
	(
	select Company_Name,Product_Name,Qty=sum(Damage_Qty)
	from tbl_damage
	group by Company_Name, Product_name
	) c
	on a.Company_Name=c.Company_Name
	and a.Product_Name=c.Product_Name
	    
	<pre>	where stcok <> 0