mysql - Sql union 操作

Question

问题：
第一种写法：

(select  du.day,du.apptoken ,du.version, du.channel,du.city,du.count,concat(apptoken, 

version,channel,city) as joinkey from day_new_users_count du where day='20170319') as dayUsers 

 union 

(select tu.day,tu.apptoken,tu.version,tu.channel,tu.city,tu.count,concat(apptoken,version,channel,city) as joinkey from total_users tu where day='20170318') as toUsers

第二种写法：

select  du.day,du.apptoken ,du.version, du.channel,du.city,du.count,concat(apptoken, version,channel,city) as joinkey from day_new_users_count du where day='20170319'
 union 

select tu.day,tu.apptoken,tu.version,tu.channel,tu.city,tu.count,concat(apptoken,version,channel,city) as joinkey from total_users tu where day='20170318'

为什么第二种写法可以正确执行，第一种方式就不可以？？

区别 不是 第一种方式中给 临时表起了个别名嘛，怎么就不行了？高人指点呐

Answer 1

select * from (
select
du.day, du.apptoken , du.version, du.channel, du.city, du.count,
concat(apptoken, version,channel,city) as joinkey
from day_new_users_count du
where day='20170319'
) as dayUsers
union
select * from (
select
tu.day,  tu.apptoken, tu.version, tu.channel, tu.city, tu.count,
concat(apptoken,version,channel,city) as joinkey
from total_users tu
where day='20170318'
) as toUsers