Order of inner join - sql

Question

I was inspecting Northwind database. There is one View called 'Invoice' fairly complicated.
I tried to change the original sql (pull last inner join up, merge with the other inner join statement).
'Official' code to get invoice: (correct)
SELECT *
FROM dbo.Shippers as shipper
INNER JOIN dbo.Products as product
INNER JOIN dbo.Employees as employee
INNER JOIN dbo.Customers as customer
INNER JOIN orders as orders
ON customer.CustomerID = orders.CustomerID
ON employee.EmployeeID = orders.EmployeeID
INNER JOIN dbo.[Order Details]
ON orders.OrderID = dbo.[Order Details].OrderID
ON product.ProductID = dbo.[Order Details].ProductID
ON shipper.ShipperID = orders.ShipVia
My first try: (doesn't work)
SELECT *
FROM dbo.Shippers as shipper
INNER JOIN dbo.Products as product
INNER JOIN dbo.Employees as employee
INNER JOIN dbo.Customers as customer
INNER JOIN orders as orders
INNER JOIN dbo.[Order Details]
ON orders.OrderID = dbo.[Order Details].OrderID
ON product.ProductID = dbo.[Order Details].ProductID
ON shipper.ShipperID = orders.ShipVia
ON customer.CustomerID = orders.CustomerID
ON employee.EmployeeID = orders.EmployeeID
My second try (works):
select *
from Orders as orders
inner join Shippers as ships
on ships.ShipperID = orders.ShipVia
inner join [Order Details] as ods
on ods.OrderID = orders.OrderID
inner join Products as products
on ods.ProductID = products.ProductID
inner join Customers as customers
on customers.CustomerID = orders.CustomerID
Both of them return 2155 lines of record.
Here is a reference for Table Structure:
My question is why 'my first try' is not right ?
Also, do you think my second try is right ?
Thanks

Answer 1

Each ON clause is used to specify the join conditions for the most immediate preceding JOIN clause that hasn't yet had an ON specified.
So, indenting to show how they're matching up:
SELECT *
FROM dbo.Shippers as shipper
INNER JOIN dbo.Products as product
INNER JOIN dbo.Employees as employee
INNER JOIN dbo.Customers as customer
INNER JOIN orders as orders
INNER JOIN dbo.[Order Details]
ON orders.OrderID = dbo.[Order Details].OrderID
ON product.ProductID = dbo.[Order Details].ProductID
ON shipper.ShipperID = orders.ShipVia
ON customer.CustomerID = orders.CustomerID
ON employee.EmployeeID = orders.EmployeeID
And the most deeply nested JOIN will be performed first. So, whilst the inner most join seems to be correct (joining orders to Order Details with an ON clause of orders.OrderID = dbo.[Order Details].OrderID), the next one out is incorrect - we're trying to join customer to the result of the previous join (or orders and order details), but with an ON clause of product.ProductID = dbo.[Order Details].ProductID - and that's wrong because we've not joined to the product table yet.
You might try re-arranging them as:
SELECT *
FROM dbo.Shippers as shipper
INNER JOIN dbo.Products as product
INNER JOIN dbo.Employees as employee
INNER JOIN dbo.Customers as customer
INNER JOIN orders as orders
INNER JOIN dbo.[Order Details]
ON orders.OrderID = dbo.[Order Details].OrderID
ON customer.CustomerID = orders.CustomerID
ON employee.EmployeeID = orders.EmployeeID
ON product.ProductID = dbo.[Order Details].ProductID
ON shipper.ShipperID = orders.ShipVia
Where now at least each ON clause is dealing in terms of table aliases that are in scope for each join.
However, I'd usually recommend (except for complex joins) to follow a pattern of:
FROM a
INNER JOIN b
ON a.column = b.column
INNER JOIN c
ON a_or_b.column = c.column
...
Where each ON clause is kept close to the JOIN clause that it's actually specifying the conditions for, much like your second attempt. I can see no reason for trying to get all of the JOINs to occur at the top of the FROM.

Answer 2

in your 1st try, you didn't link Shipper to Order table.
SELECT *
FROM dbo.Shippers as shipper
INNER JOIN dbo.Products as product
INNER JOIN dbo.Employees as employee
INNER JOIN dbo.Customers as customer
INNER JOIN orders as orders
INNER JOIN dbo.[Order Details]
ON orders.OrderID = dbo.[Order Details].OrderID
ON product.ProductID = dbo.[Order Details].ProductID
ON shipper.ShipperID = orders.ShipVia
ON customer.CustomerID = orders.CustomerID
ON employee.EmployeeID = orders.EmployeeID
2ndly when you use join, you have to mention the columns of the two table on which the join will be done.
so when you join x and join y then you have to mention column of x and column of y. otherwise it will be a Cartesian product. when you make a join just immediately you have to mention the column.
in your second query, the ideal practice is link all the associated table 1st.
you can re-write it like:
select *
from Orders as orders
inner join Shippers as ships
on orders.ShipVia= ships.ShipperID
inner join Customers as customers
on orders.CustomerID = customers.CustomerID
inner join [Order Details] as ods
on orders.OrderID = ods.OrderID
inner join Products as products
on ods.ProductID = products.ProductID