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

In regards to adding an ORDER BY clause:

This is probably elementary to most here but I thought I add this. Sometimes you don't want the results mixed, so you want the first query's results then the second and so on. To do that I just add a dummy first column and order by that. Because of possible issues with forgetting to alias a column in unions, I usually use ordinals in the order by clause, not column names.

For example:

SELECT 1, * FROM xxx WHERE z = 'abc'
UNION ALL
SELECT 2, * FROM xxx WHERE z = 'def'
UNION ALL
SELECT 3, * FROM xxx WHERE z = 'ghi'
ORDER BY 1

The dummy ordinal column is also useful for times when I'm going to run two queries and I know only one is going to return any results. Then I can just check the ordinal of the returned results. This saves me from having to do multiple database calls and most empty resultset checking.