Question

Can someone please identify the functional/performance differences, if any, between SET and SELECT in T-SQL? Under what conditions should I choose one over the other?

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UPDATE:
Thanks to all who responded. As a few people pointed out, this article by Narayana Vyas Kondreddi has lots of good info. I also perused the net after reading the article and found this condensed version by Ryan Farley which offers the highlights and thought I would share them:

1. SET is the ANSI standard for
   variable assignment, SELECT is not.
2. SET can only assign one variable at
   a time, SELECT can make multiple
   assignments at once.
3. If assigning from a query, SET can
   only assign a scalar value. If the
   query returns multiple values/rows
   then SET will raise an error. SELECT
   will assign one of the values to the
   variable and hide the fact that
   multiple values were returned (so
   you’d likely never know why
   something was going wrong elsewhere – have fun troubleshooting that one)
4. When assigning from a query if there
   is no value returned then SET will
   assign NULL, where SELECT will not
   make the assignment at all (so the
   variable will not be changed from
   it’s previous value)
5. As far as speed differences – there
   are no direct differences between
   SET and SELECT. However SELECT’s
   ability to make multiple assignments
   in one shot does give it a slight
   speed advantage over SET.

Answer 1

SET is the ANSI standard way of assigning values to variables, and SELECT is not. But you can use SELECT to assign values to more than one variable at a time. SET allows you to assign data to only one variable at a time. So that in performance is where SELECT will be a winner.

For more detail and examples refer to: Difference between SET and SELECT when assigning values to variables