SQL rowid 和 rownum

rowid和rownum都是虚列，但含义完全不同。rowid是物理地址，用于定位oracle中具体数据的物理存储位置，而rownum则是sql的输出结果排序，从下面的例子可以看出其中的区别。

SQL> select rowid,deptno from dept order by deptno;

ROWID DEPTNO
------------------ ----------
AAABe/AAFAAABLqAAA 10
AAABe/AAFAAABLqAAB 20
AAABe/AAFAAABLqAAC 30
AAABe/AAFAAABLqAAD 40

SQL> edit
Wrote file afiedt.buf

1* select rowid,deptno from dept order by deptno desc
SQL> /

ROWID DEPTNO
------------------ ----------
AAABe/AAFAAABLqAAD 40
AAABe/AAFAAABLqAAC 30
AAABe/AAFAAABLqAAB 20
AAABe/AAFAAABLqAAA 10

SQL> edit
Wrote file afiedt.buf

1* select rownum,deptno from dept order by deptno
SQL> /

ROWNUM DEPTNO
---------- ----------
1 10
2 20
3 30
4 40

SQL> edit
Wrote file afiedt.buf

1* select rownum,deptno from dept order by deptno desc
SQL> /

ROWNUM DEPTNO
---------- ----------
1 40
2 30
3 20
4 10

 

just think about ROWNUM pseudo column as a 'magic'. You can do

where rownum < n

or

where rownum = 1

but NOT

where rownum > n

or

where rownum = some number > 1

If you have to you can do something like:

select * from(
select t.*,rownum rn from t
)
where rn = 3