问一下这条sql，为什么不能这么写？

我想实现下面这个功能：
SELECT DONE, (SELECT COUNT(*) FROM TABLE2) AS SUCCESS, CREATETIME
  FROM (SELECT SUM(OFFSET) AS DONE, SYSDATE AS CREATETIME FROM TABLE1)
 问一下，为什么不能这么写？
SQL code

select sum(offset) as done,
       (select count(*) from table2) as success,
       sysdate as createtime
  from table1

用了聚集函数sum，就不能在查询其他列如sysdate，否则如何将sum(offset)这个值与多个sysdate的值对应起来？
select （select sum(offset) from table1） as done,
  (select count(*) from table2) as success,
  sysdate as createtime
from table1

SQL code

select sum(offset) as done,
       (select count(*) from table2) as success,   ----该处:非单组分组函数
       sysdate as createtime
  from table1
--要增加分组函数
group by

--考虑子查询不能GROUP BY

SQL code

select sum(offset) as done,
       (select count(*) from table2) as success,
       sysdate as createtime
  from table1
group by sysdate

子查询不能GROUP BY,哦，这样啊，谢谢