We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SELECT 2 UNION
SELECT 1 union
SELECT NULL UNION 
SELECT '1' union
SELECT '2' 

SELECT 2 UNION
select a
from 
( 
SELECT '2' a UNION ALL 
SELECT 1 
) q UNION
SELECT NULL UNION 
SELECT '1' 

option (merge union)

SELECT 2 UNION
select a
from ....

option (hash union)