How To Test If Record Is Found Or Not And Do Different Things Accordingly In A Query

 I want to do something like
if "Product_code" is Null
insert dbo.t_Shopping_cart (Product_code,Name,Price,Product_group,Quantity,Total)
select Product_code,Name,Price,Product_group,Quantity=1,Yhteensa=1
FROM dbo.t_Shopping_cart
WHERE Product_code='AHTU140213'
else
update dbo.t_Shopping_cart
set Quantity=Quantity+1
where Product_code='AHTU140213'
 
In short: I cant test existence of a record.
Regards
Leif