Triggers Inner join inserted with orginial table - sql-server

Question

I was reviewing creating DML triggers in SQL Server in SQL docs: Use the inserted and deleted Tables
There is an example which do the following:
The following example creates a DML trigger. This trigger checks to make sure the credit rating for the vendor is good when an attempt is made to insert a new purchase order into the PurchaseOrderHeader table. To obtain the credit rating of the vendor corresponding to the purchase order that was just inserted, the Vendor table must be referenced and joined with the inserted table. If the credit rating is too low, a message is displayed and the insertion does not execute.
Note that this example does not allow for multirow data modifications.
USE AdventureWorks2012;
GO
IF OBJECT_ID ('Purchasing.LowCredit','TR') IS NOT NULL
DROP TRIGGER Purchasing.LowCredit;
GO
-- This trigger prevents a row from being inserted in the Purchasing.PurchaseOrderHeader table
-- when the credit rating of the specified vendor is set to 5 (below average).
CREATE TRIGGER Purchasing.LowCredit
ON Purchasing.PurchaseOrderHeader
AFTER INSERT
AS
IF EXISTS (SELECT *
FROM Purchasing.PurchaseOrderHeader p
JOIN inserted AS i ON p.PurchaseOrderID = i.PurchaseOrderID
JOIN Purchasing.Vendor AS v ON v.BusinessEntityID = p.VendorID
WHERE v.CreditRating = 5)
BEGIN
RAISERROR ('A vendor''s credit rating is too low to accept new purchase orders.', 16, 1);
ROLLBACK TRANSACTION;
RETURN
END;
GO
I wonder why the example inner joined the inserted table with Purchasing.PurchaseOrderHeader table and then join the vendor table.
Can I get the same result using only the inserted table joining the vendor table directly without joining with Purchasing.PurchaseOrderHeader table?

Answer 1

Not only do I believe you are correct, I think there are further inaccuracies on this documentation page - which I have to admit is pretty rare in my experience.
Take for example this part:
Note that this example does not allow for multirow data modifications.
This is a false claim. The trigger code example will handle multiple rows insert as well as a single row insert.
Note that according to the documented (and observed) behavior, the inserted table will contain all the rows inserted (or updated) to the trigger's target table:
The inserted table stores copies of the affected rows during INSERT and UPDATE statements. During an insert or update transaction, new rows are added to both the inserted table and the trigger table. The rows in the inserted table are copies of the new rows in the trigger table.
Therefor, the join to inserted should be enough in this case to enforce the business rule discussed.
That being said, using triggers to enforce business rules might prove difficult and even problematic - note that this trigger only covers inserted rows, but not updated rows. This means that a new row might be inserted with valid values, and later on updated to invalid values.