SQL Server UNION - What is the default ORDER BY Behaviour

Question

If I have a few UNION Statements as a contrived example:

SELECT * FROM xxx WHERE z = 1
UNION 
SELECT * FROM xxx WHERE z = 2
UNION
SELECT * FROM xxx WHERE z =         


        

Answer 1

A UNION can be deceptive with respect to result set ordering because a database will sometimes use a sort method to provide the DISTINCT that is implicit in UNION , which makes it look like the rows are deliberately ordered -- this doesn't apply to UNION ALL for which there is no implicit distinct, of course.

However there are algorithms for the implicit distinct, such as Oracle's hash method in 10g+, for which no ordering will be applied.

As DJ says, always use an ORDER BY