聚集函數內的子查詢-出錯

--1. 無法執行
select sum(
       select n from 	  --ORA-00936: 缺失表達式
       (select to_number('1') as n from dual
       union all
       select to_number('2') as n from dual)
) from dual

--2. 可以執行
select sum(n) 
from
 (select n from
       (select to_number('1') as n from dual
       union all
       select to_number('2') as n from dual
       )
)

哪位仁兄給[b][u]詳細[/u][/b]解釋一下，爲什麼第1種情況在oracle裏會報錯？或提供可參考的資料 OTZ