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

There is no default order.

Without an Order By clause the order returned is undefined. That means SQL Server can bring them back in any order it likes.

EDIT: Based on what I have seen, without an Order By, the order that the results come back in depends on the query plan. So if there is an index that it is using, the result may come back in that order but again there is no guarantee.