Question

When running the following query

UPDATE Invoices
SET PaymentTotal = 1
FROM Vendors
WHERE Vendors.VendorID = 34 -- true for some row

SQL Server updates all records in Invoices. However, when the condition does not match any row in the Vendors table:

UPDATE Invoices
SET PaymentTotal = 1
FROM Vendors
WHERE Vendors.VendorID = -34 -- false for all rows

no rows get updated. WHY?

Edit: This is a COMPLETELY academic question and I understand that this is not the ideal way to write queries.

Answer 1

Your original query:

UPDATE Invoices
SET PaymentTotal = 1
FROM Vendors
WHERE Vendors.VendorID = 34

is equivalent to:

update i set paymenttotal = 1
from vendors v
cross join invoices i
where v.vendorid = 34

the cross join returns the product of the two sets and when vendors is reduced to an empty set with the criteria v.vendorid = -34 then the resulting set is empty because anything multiplied by zero is zero.