Oracle update和order by

今天遇到一个关于SQL转换成Oracle语句的问题，描述如下：

select * from emp order by deptno;

select * from dept;

Sql Server:

update dept a set dname=(select top 1 ename from emp where deptno=a.deptno order by sal)

经过尝试，查找资料，得出下面转换结果，不知道这样是否可行：

update dept a set dname=

(with t as(select ename,deptno from emp order by sal)

select ename from t where deptno=a.deptno and rownum=1)

where exists(with t as(select ename,deptno from emp order by sal)

select null from t where deptno=a.deptno)

执行结果：

select * from dept;

cost:22

另外一种：

update dept a
set dname =
(select max(ename)
from emp
where deptno = a.deptno
and sal = (select min(sal) from emp where deptno=a.deptno))
where exists (select null
from emp
where deptno = a.deptno);

这种耗cost比第一个更大。

cost:27

第三种：

update dept a
set dname =
(select col
from (select decode(row_number()
over(partition by deptno order by sal),
1,
ename,
null) col,
deptno
from emp) b
where b.deptno = a.deptno
and b.col is not null)
where exists (select null from emp where deptno = a.deptno);

cost:21

已找到较简洁的方法：

update dept a
set dname =
(select max(ename) keep(dense_rank first order by nvl(sal, 0))
from emp
where deptno = a.deptno)
where exists (select null from emp where deptno = a.deptno);

cost：18