To DB/SQL Teachers: A Wrong Query On Northwind DB Discussion

Good day to all.

I'm a DB teacher in a University. Planning and developing SQL exercises for my students I found a "tricky" or extrange sql exercise.

The sql exercise using Northwind DB reads as follows:
"Wich is the total amount of the freight that corresponds to all orders containing products of seafood category ?"

The first sql code that comes to my mind was:

select sum(freight)
from orders o, "order details" d, products p, categories c
where o.orderid=d.orderid and d.productid=p.productid and p.categoryid=c.categoryid
and categoryname='seafood'

This is equivalent code constructed with Query constructor in Enterprise Manager:
SELECT SUM(dbo.Orders.Freight) AS Expr1
FROM dbo.Orders INNER JOIN
dbo.[Order Details] ON dbo.Orders.OrderID = dbo.[Order Details].OrderID INNER JOIN
dbo.Products ON dbo.[Order Details].ProductID = dbo.Products.ProductID INNER JOIN
dbo.Categories ON dbo.Products.CategoryID = dbo.Categories.CategoryID
WHERE (dbo.Categories.CategoryName = N'seafood')

These 2 equivalent queries output 27722.9600 as result, but the correct result (total freight) is 23791.1400.

Did you also think (as I did) that the exposed code is correct?

Now I know the correct sql code that gives the corect answer and also the explanation why the code exposed fails, but before share it under your posible request, want to know your tech comments about this exercise in order to know if this particular exercise is "tricky" (make people to fail) or is my total fault and need to review my strategies of applying/constructing sql (and teaching).


Thanks a lot.

Carlos