Minimum of column B that falls under maximum of column A

declare @a table(col1 datetime, col2 datetime)
insert into @a select
'20120520', '20120620' union all select
'20120520', '20120513' union all select
'20120530', '20120620' union all select
'20120530', '20120515' union all select
'20120630', '20120515' union all select
'20120630', '20120510' union all select
'20120630', '20120525' 
--First approach
SELECT	*
FROM	(
		SELECT	*, RN = ROW_NUMBER() OVER (ORDER BY col1 DESC, col2)
		FROM	@a
		)a
WHERE	RN = 1;

/ * OUTPUT 
col1	                col2	                RN
2012-06-30 00:00:00.000	2012-05-10 00:00:00.000	1
*/