I'm responsible for some old T-SQL code, and I've noticed that a few queries return incorrect result sets. The queries I'm having trouble with all use outer joins and the non-ANSI syntax for expressing them, which was required in SQL Server 6.5—before ANSI-compliant outer-join syntax was added. For example, the following query against the Northwind sample database returns incorrect results:

SELECT Customers.CustomerId
FROM customers, orders
WHERE customers.Customerid...