Sql语句中，from后面跟查询结果，这个查询结果可以用union连接吗

A\B\C三个表，A表和B表中只有一个表可以查询到registno

select registno, status from A where registno = '123456';
select registno, status from B where registno = '123456';
select pumpid from C;

我如下这么写就报错

select a.registno, b.pumpid
from 
(select registno, status from A where registno = '123456'
union
select registno, status from B where registno = '123456') a
,outer prplcompensate b
where 
a.registno=b.registno
group by 1;

错误信息：

[Code: -999, SQL State: IX000]  Not implemented yet.  [Script position: 2976 - 2957]

是我语句有问题吗？

outer去掉，或者改成left outer join
group by 1从查询结果看好像没啥作用

select a.registno, b.pumpid
from 
(select registno, status from A where registno = '123456'
union
select registno, status from B where registno = '123456') a
, prplcompensate b
where 
a.registno=b.registno
group by 1;

select a.registno, b.pumpid
from 
(select registno, status from A where registno = '123456'
union
select registno, status from B where registno = '123456') a
left outer join prplcompensate b on a.registno=b.registno
group by 1;

好像b.pumpid这个值没有用吧
select a.registno from （select registno from A where registno = '123456' union select registno from B where registno = '123456'）a left join prplcompensate b on a.registno=b.registno;

前面展示的列必须都在group by后面，不然用聚合函数。是不是这个原因？