含义解释： 
                  1、rownum是oracle系统顺序分配为从查询返回的行的编号，返回的第一行分配的是1，第二行是2，
                  　 依此类推，这个伪字段可以用于限制查询返回的总行数。
                  2、rownum不能以任何基表的名称作为前缀。 
                  使用方法： 
                  现有一个商品销售表sale，表结构为：
                  month　　　 char(6)　　　　　 --月份
                  sell　　　　number(10,2)　　　--月销售金额

                  create table sale (month char(6),sell number);
                  insert into sale values('200001',1000);
                  insert into sale values('200002',1100);
                  insert into sale values('200003',1200);
                  insert into sale values('200004',1300);
                  insert into sale values('200005',1400);
                  insert into sale values('200006',1500);
                  insert into sale values('200007',1600);
                  insert into sale values('200101',1100);
                  insert into sale values('200202',1200);
                  insert into sale values('200301',1300);
                  insert into sale values('200008',1000);
                  commit;

                  SQL>; select rownum,month,sell from sale where
                  rownum=1;（可以用在限制返回记录条数的地方，保证不出错，如：隐式游标）

                     ROWNUM MONTH       SELL
                  --------- ------ ---------
                          1 200001      1000

                  SQL>; select rownum,month,sell from sale where
                  rownum=2;（1以上都查不到记录）

                  没有查到记录

                  SQL>; select rownum,month,sell from sale where rownum>;5;
                  （由于rownum是一个总是从1开始的伪列，Oracle 认为这种条件不成立，查不到记录）

                  没有查到记录

                  只返回前3条纪录
                  SQL>; select rownum,month,sell from sale where rownum<4;

                     ROWNUM MONTH       SELL
                  --------- ------ ---------
                          1 200001      1000
                          2 200002      1100
                          3 200003      1200

                  如何用rownum实现大于、小于逻辑？（返回rownum在4—10之间的数据）（minus操作，速度会受影响）
                  SQL>; select rownum,month,sell from sale where rownum<10
                    2  minus
                    3  select rownum,month,sell from sale where rownum<5;

                     ROWNUM MONTH       SELL
                  --------- ------ ---------
                          5 200005      1400
                          6 200006      1500
                          7 200007      1600
                          8 200101      1100
                          9 200202      1200

                  想按日期排序，并且用rownum标出正确序号（有小到大）
                  SQL>; select rownum,month,sell from sale order by month;

                     ROWNUM MONTH       SELL
                  --------- ------ ---------
                          1 200001      1000
                          2 200002      1100
                          3 200003      1200
                          4 200004      1300
                          5 200005      1400
                          6 200006      1500
                          7 200007      1600
                         11 200008      1000
                          8 200101      1100
                          9 200202      1200
                         10 200301      1300

                  查询到11记录.

                  可以发现，rownum并没有实现我们的意图，系统是按照记录入库时的顺序给记录排的号，rowid也是顺序分配的

                  SQL>; select rowid,rownum,month,sell from sale order by rowid;

                  ROWID                 ROWNUM MONTH       SELL
                  ------------------ --------- ------ ---------
                  000000E4.0000.0002         1 200001      1000
                  000000E4.0001.0002         2 200002      1100
                  000000E4.0002.0002         3 200003      1200
                  000000E4.0003.0002         4 200004      1300
                  000000E4.0004.0002         5 200005      1400
                  000000E4.0005.0002         6 200006      1500
                  000000E4.0006.0002         7 200007      1600
                  000000E4.0007.0002         8 200101      1100
                  000000E4.0008.0002         9 200202      1200
                  000000E4.0009.0002        10 200301      1300
                  000000E4.000A.0002        11 200008      1000

                  查询到11记录.

                  正确用法，使用子查询
                  SQL>; select rownum,month,sell from (select month,sell from
                  sale group by month,sell) where rownum<13;

                     ROWNUM MONTH       SELL
                  --------- ------ ---------
                          1 200001      1000
                          2 200002      1100
                          3 200003      1200
                          4 200004      1300
                          5 200005      1400
                          6 200006      1500
                          7 200007      1600
                          8 200008      1000
                          9 200101      1100
                         10 200202      1200
                         11 200301      1300

                  按销售金额排序，并且用rownum标出正确序号（有小到大）
                  SQL>; select rownum,month,sell from (select sell,month from
                  sale group by sell,month) where rownum<13;

                     ROWNUM MONTH       SELL
                  --------- ------ ---------
                          1 200001      1000
                          2 200008      1000
                          3 200002      1100
                          4 200101      1100
                          5 200003      1200
                          6 200202      1200
                          7 200004      1300
                          8 200301      1300
                          9 200005      1400
                         10 200006      1500
                         11 200007      1600

                  查询到11记录.

                  利用以上方法，如在打印报表时，想在查出的数据中自动加上行号，就可以利用rownum。

                  返回第5—9条纪录，按月份排序
                  SQL>; select * from (select rownum row_id ,month,sell 
                    2  from (select month,sell from sale group by month,sell)) 
                    3  where row_id between 5 and 9;

                      ROW_ID MONTH        SELL
                  ---------- ------ ----------
                           5 200005       1400
                           6 200006       1500
                           7 200007       1600
                           8 200008       1000
                           9 200101       1100