Stewart "Arturius" Campbell (11/25/2011)
This was quite a tough one.Learned a fair bit about the origin of the MERGE statement from this.
I definitely second this.
Some may argue it's cheating to do some reading (researching) prior to answering the QotD, but I came across this interesting and helpful blog post by one Hugo Kornelis :-).
Hence a double thank you to Hugo for an excellent QotD on MERGE following the ANSI standard, and for sharing your extensive knowledge with everybody on the web.
You all have a great weekend,
Michael