Use a calculated column in a where clause

You'll need to wrap the inner query in a derived table or CTE in order to be able to use derived columns in the WHERE clause (Also, note SUM() is specified just once, using the results of the multiplication):

SELECT x.Code, x.AccountNumber, x.SalesFROM(  SELECT p.Code, c.AccountNumber, SUM(p.UnitPrice *od.QtyShipped) AS Sales   FROM [dbo].Customer c       LEFT JOIN [dbo].OrderHeader oh ON oh.CustomerId = c.Id       LEFT JOIN [dbo].OrderDetail od ON od.OrderHeaderId = oh.Id      LEFT JOIN [dbo].Product p ON p.Id = od.ProductId  GROUP BY p.Code, c.AccountNumber) AS xWHERE x.Sales > 100;

You can use the common table expression for this

;WITH CTE AS    (    SELECT p.Code, c.AccountNumber, Sales = (SUM(p.UnitPrice) * SUM(od.QtyShipped)) FROM [dbo].Customer c         LEFT JOIN [dbo].OrderHeader oh ON oh.CustomerId = c.Id         LEFT JOIN [dbo].OrderDetail od ON od.OrderHeaderId = oh.Id        LEFT JOIN [dbo].Product p ON p.Id = od.ProductId        GROUP BY p.Code, c.AccountNumber, Sale        )    SELECT *FROM CTE WHERE CTE.Sales>100

If it's a calculated column you can use "HAVING".

SELECT p.Code, c.AccountNumber, Sales = (SUM(p.UnitPrice) * SUM(od.QtyShipped)) FROM [dbo].Customer c     LEFT JOIN [dbo].OrderHeader oh ON oh.CustomerId = c.Id     LEFT JOIN [dbo].OrderDetail od ON od.OrderHeaderId = oh.Id    LEFT JOIN [dbo].Product p ON p.Id = od.ProductIdGROUP BY p.Code, c.AccountNumber, SalesHAVING SALES > 100;