Add optional support for the EXCEPT operator in the WHEN MATCHED clause

[jwk6]

Add optional support for the EXCEPT operator in the WHEN MATCHED clause, as described here:
http://www.made2mentor.com/2013/05/writing-t-sql-merge-statements-the-right-way/

For Example:
WHEN MATCHED AND EXISTS
(SELECT Source.Column1, Source.Column2
EXCEPT
SELECT Target.Column1, Target.Column2)

This will properly handle NULL values in both the source and target, and will be a little more clear and concise for the developer.

EXCEPT is supported in all currently supported versions of SQL Server, and in Azure SQL Database.
https://docs.microsoft.com/en-us/sql/t-sql/language-elements/set-operators-except-and-intersect-transact-sql?view=sql-server-ver15

[NowinskiK]

Interesting idea!

[jwk6]

@NowinskiK I'm feeling the urge to do something again. Mind if I work on this one?