oracle 别名排序

select expr as 别名 from tablename
order by 别名 desc
或者
select 别名=expr from tablename
order by 别名 desc

select expr from tablename order by 1 desc
按第一列降序,意思和select expr from tablename order by expr desc一样
select a,b,c from tablename
order by a,c desc,b
意思是先按a升序，在按c降序，在按b升序，asc可以不加