oracle 中rownum row_number() over() 的对比
--创建表
create table T_NEWS
(
  ID      NUMBER,
  N_TYPE  VARCHAR2(20),
  N_TITLE VARCHAR2(30),
  N_COUNT NUMBER
)

--插入数据
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (1, 'IT', '爱it1', 100);
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (2, '体育', '爱体育1', 10);
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (3, '体育', '爱体育2', 30);
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (4, 'IT', '爱it2', 300);
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (5, 'IT', '爱it3', 200);
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (6, '体育', '爱体育3', 20);
insert into T_NEWS (ID, N_TYPE, N_TITLE, N_COUNT)
values (7, '体育', '爱体育4', 60);
commit;


--分页 row_number，不是rownum
--根据n_count从大到小排列，每页3条
SELECT ROWNUM r,t.* FROM t_news t
WHERE ROWNUM<=3
ORDER BY t.n_count DESC
--问题：为什么order by以后，行号是乱的？
SELECT ROWNUM r,t.* FROM t_news t
--原因：先分配了行号，再根据n_count排序

--所以必须排序，再生成行号
SELECT ROWNUM r,t.* FROM (
   SELECT t.* FROM t_news t ORDER BY t.n_count DESC  ) t
  
--分页
--err
SELECT ROWNUM r,t.* FROM (
   SELECT t.* FROM t_news t ORDER BY t.n_count DESC  ) t
WHERE r between 1 AND 3


--第1页
SELECT ROWNUM r,t.* FROM (
   SELECT t.* FROM t_news t ORDER BY t.n_count DESC  ) t
WHERE ROWNUM between 1 AND 3

--第2页
SELECT ROWNUM r,t.* FROM (
   SELECT t.* FROM t_news t ORDER BY t.n_count DESC  ) t
WHERE ROWNUM between 4 AND 6
--error: ROWNUM必须从1开始！
SELECT k.* FROM (
SELECT ROWNUM r,t.* FROM (
   SELECT t.* FROM t_news t ORDER BY t.n_count DESC  ) t
) k
WHERE r BETWEEN 4 AND 6


-------第二步:我用row_number() over()函数
select t2.*
  from (select t.*, row_number() over(order by t.n_count desc) orderNumber
          from t_news t
         order by t.n_count desc) t2
where orderNumber between 1and 3;