Using union to sort MySQL

EXPLAIN SELECT * from  (


select * from ( SELECT
	* 
FROM
	article 
WHERE
	top_start_time <= now() AND top_end_time >= now() 
	AND show_status = 1 
	AND top_status = 1 
	AND mark > 0 ORDER BY top_sort_num asc     limit 100000 ) 
	as t1

UNION

select * from  (

SELECT
	* 
FROM
	article 
WHERE
	    show_status = 1 
	AND mark > 0  ) as t2

) as t 
 
 	ORDER BY create_time desc 
	
	LIMIT 100