L' Eomot Inversé (12/22/2012)
Good question.But the explanation, as it refers to nullif and to view5, is a bit of a mess - it appears to say that view5 will always return null, instead of an error, when the divisor is zero, which is wrong (and conflicts with the answer given, which is right). The thing about nullif (as used in view5) is that it is equivalent to a two-branch case statement, and almost always both branches will be executed in currently supported systems, even if the divisor is zero.
Edit: the current implementation of CASE can (and does) use eager rather than lazy evaluation, which means that it can't be used to eliminate errors by having separate branches for error-producing and safe states; and since nullif is just a shorthand for a case statement, it has the same issue. I regard this as an example of an opportunity for optimisation being used as an excuse to destroy the apparent semantics of the language, which is of course inexcusable.
As you can see from my previous reply to this topic, I disagree with your comment.
I think you are confusing two things about processing and evaluation order in a CASE expression. Let's look at an example:
CASE
WHEN x <> 0 THEN y / x
WHEN (very complicated subquery expression) = 1 THEN (yet another very complicated subquery)
ELSE 42
END
SQL Server guarantees that "y/x" will only be actually computed when "x <> 0" evaluates to True. It also guarantees that "yet another very complicated subquery" will only be actually computed when "x <> 0" does not, and "(very complicated subquery expression) = 1" does evaluate to True.
Some people also expect that SQL Server will not even process (very complicated subquery expression) when "x <> 0" evaluates to true. And while that could have been the case (after all, the result of that subquery evaluation will not be used), this, in fact, is not guaranteed. My suspicion after reading your comment is that you either once read, or once were bitten by this latter behaviour, and then erroneoulsy expanded into no longer trusting SQL Server not to evaluate the THEN if the WHEN does not evaluate to True.