SQL用了Union后的排序问题

1.UNION查询没问题

SELECT `id`,`username`,`mobile`,`time`,id AS leader 
FROM `grouporder_leader` 
WHERE `courseid` = 21 AND `merchid` = 23 AND `status` = 1 
UNION ALL 
SELECT leadorderid,username,mobile,time,null 
FROM `grouporder_partner`
WHERE courseid=21 and status=1 and merchid=23

2.排序就出问题了

SELECT `id`,`username`,`mobile`,`time`,id AS leader 
FROM `grouporder_leader`
WHERE `courseid` = 21 AND `merchid` = 23 AND `status` = 1 
ORDER BY time DESC 
UNION ALL 
SELECT leadorderid,username,mobile,time,null 
FROM `grouporder_partner`
WHERE courseid=21 and status=1 and merchid=23 
ORDER BY time DESC

使用类似于创建临时表的方法保存查询结果，然后对临时表进行查询排序。

SELECT id,username,mobile,time,leader 
FROM (SELECT `id`,`username`,`mobile`,`time`,id AS leader 
	FROM `grouporder_leader` WHERE `courseid` = 21 AND `merchid` = 23 AND `status` = 1 
	UNION ALL 
	SELECT leadorderid,username,mobile,time,null 
	FROM `grouporder_partner` WHERE courseid=21 and status=1 and merchid=23
) 
ORDER BY time DESC

不知道为什么第3步中查询依旧没有，然后对UNION查询的结果起个别名，然后再查询排序就没问题了。

SELECT a.id,a.username,a.mobile,a.time,a.leader 
FROM (SELECT `id`,`username`,`mobile`,`time`,id AS leader 
	FROM `grouporder_leader` WHERE `courseid` = 21 AND `merchid` = 23 AND `status` = 1 
	UNION ALL 
	SELECT leadorderid,username,mobile,time,null 
	FROM `grouporder_partner` WHERE courseid=21 and status=1 and merchid=23
) AS a 
ORDER BY time DESC

查出来就好说了,再进行去重或者其他操作,也没问题.

SELECT DISTINCT a.id,a.username,a.mobile,FROM_UNIXTIME(a.time,%Y/%m/%d) as _time,a.leader 
FROM (SELECT `id`,`username`,`mobile`,`time`,id AS leader 
	FROM `grouporder_leader` WHERE `courseid` = 21 AND `merchid` = 23 AND `status` = 1 
	UNION ALL 
	SELECT leadorderid,username,mobile,time,null 
	FROM `grouporder_partner` WHERE courseid=21 and status=1 and merchid=23
) AS a 
ORDER BY time DESC