ORACLE中的ROWNUM?

[redliu2004]

下面是一些关于ROWNUM的疑问,谢谢哪位专家给我解答一下:
表T(C1,C2,C3)

1)SELECT C1,C2,C3 ,ROWNUM FROM T WHERE ROWNUM < 3;
却不能写成:
SELECT C1,C2,C3,T.ROWNUM FROM T WHERE T.ROWNUM < 3;

2)为什么下面要用别名,否则会出现语法错误
SELECT * FROM
(SELECT ROWNUM AS R FROM T )D
WHERE D.R < 3;

[ThunkYao]

1。ROWNUM 是记录的条数，并不是表的字段

[welyngj]

1）rownum不是表的字段。
For each row returned by a query, the ROWNUM pseudocolumn returns a number indicating the order in which Oracle selects the row from a table or set of joined rows.

[welyngj]

2）如果不用表的别名,就不知道是t表的rownum还是d表的rownum

[beckhambobo]

SQL> select 1 from (select rownum r from emp) where r<=1;

1
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1

楼主基础还是不扎实