SQL用了Union后的排序问题

最近使用SQL语句进行UNION查询，惊奇的发现：SQL没问题，UNION查询也没问题，都可以得到想要的结果，可是在对结果进行排序的时候，却出问题了。

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

结果如下



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

执行这条SQL语句之后就报错。

3.创建临时表
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

4.起别名
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

结果就正确了



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