SQL Server Update with Inner Join

Question

I have 3 tables (simplified):

 tblOrder(OrderId INT)  
  tblVariety(VarietyId INT,Stock INT)  
  tblOrderItem(OrderId,VarietyId,Quantity INT)

If I place an order, I drop the stock level using this:

UPDATE tblVariety  
SET tblVariety.Stock = tblVariety.Stock - tblOrderItem.Quantity  
FROM tblVariety  
INNER JOIN tblOrderItem ON tblVariety.VarietyId = tblOrderItem.VarietyId  
INNER JOIN tblOrder ON tblOrderItem.OrderId = tblOrder.OrderId  
WHERE tblOrder.OrderId = 1

All fine, until there are two rows in tblOrderItem with the same VarietyId for the same OrderId. In this case, only one of the rows is used for the stock update. It seems to be doing a GROUP BY VarietyId in there somehow.

Can anyone shed some light? Many thanks.

SQL Server Update with Inner Join

3 Answers