如何在没有ORDER BY的情况下在UNION中订购条目？

SELECT col
FROM 
   (
       SELECT a col, 0 ordinal FROM A LIMIT 1
       UNION ALL
       SELECT b, 1 FROM B LIMIT 1
   ) t
ORDER BY ordinal

我不认为订单是保证的,至少不是所有DBMS.

我过去为控制UNION中的顺序所做的是:

(SELECT a, 0 AS Foo FROM A LIMIT 1)
UNION
(SELECT b, 1 AS Foo FROM B LIMIT 1)
ORDER BY Foo