oracle-union order by rownum排序与取数

order by在union的排序可以写在后一个select里
SELECT 5 Id,2 Name From dual
UNION ALL
Select 3,4 From dual
Order By Id

rownum在union内部只是作为后一个select的语句与前一个语句块无关，得到的数据有两条
Select Id,Name From (SELECT 1 Id,2 Name From dual
UNION ALL
Select 3,4 From dual
WHERE ROWNUM <= 1)

在语句块外面，得到的数据为一条
Select Id,Name From (SELECT 1 Id,2 Name From dual
UNION ALL
Select 3,4 From dual
)
WHERE ROWNUM <= 1

select id,name
from(SELECT 5 Id,2 Name From dual
UNION ALL
Select 3,4 From dual
Order By Id desc)
WHERE ROWNUM <= 1