oracle over函数用法

1，初始化脚本

create table emp(deptno varchar2(10),ename varchar2(20),sal number(10,0));

  insert into emp(deptno,ename,sal)values('20',trim('ADAMS   '),       1100   );
  insert into emp(deptno,ename,sal)values('30',trim('ALLEN   '),       1600   );
  insert into emp(deptno,ename,sal)values('30',trim('BLAKE   '),       2850   );
  insert into emp(deptno,ename,sal)values('10',trim('CLARK   '),       2450   );
  insert into emp(deptno,ename,sal)values('20',trim('FORD    '),      3000    );
  insert into emp(deptno,ename,sal)values('30',trim('JAMES   '),       950    );
  insert into emp(deptno,ename,sal)values('20',trim('JONES   '),       2975   );
  insert into emp(deptno,ename,sal)values('10',trim('KING    '),      5000    );
  insert into emp(deptno,ename,sal)values('30',trim('MARTIN  '),         1250 );   
  insert into emp(deptno,ename,sal)values('10',trim('MILLER  '),         1300 );   
  insert into emp(deptno,ename,sal)values('20',trim('SCOTT   '),       3000   );
  insert into emp(deptno,ename,sal)values('20',trim('SMITH   '),       800    );
  insert into emp(deptno,ename,sal)values('30',trim('TURNER  '),         1500 );  
  insert into emp(deptno,ename,sal)values('30',trim('WARD    '),      1250    );

2，具体用法

select * from (select deptno,
       ename,
       sal,
       --sum(sal) over(partition by deptno order by sal) --先按deptno分组，然后在分组内按sal排序逐个累加sal
       --sum(sal) over(order by deptno,ename)--按deptno，ename排序，逐个累加sal
       --sum(sal) over(order by ename)--按ename排序，逐个累加sal
       --sum(sal) over() --相当于sum(sal)
       --sum(sal) over (partition by deptno order by ename desc,sal desc)--先按deptno分组，然后在分组内按ename,sal倒序排序逐个累加sal
       row_number() over(/*partition by deptno*/ order by sal desc) row_n --按sal倒序排序得到行号
  from emp )
  where row_n between 1 and 3; --分页查询

3，分页方法

-- 正确方法1
select *
  from (select rownum row_num, a.*
          from (select t.* from emp t order by t.sal) a) b
 where b.row_num between 1 and 3;

-- 正确方法2，比方法1好一些
select *
  from (select rownum row_num, a.*
          from (select t.* from emp t order by t.sal) a
         where rownum <= 3) b
 where b.row_num >= 1;

-- 正确方法3，这三种分页方法性能几乎相等，如果非要分个高低：方法2 > 方法1 > 方法3，但方法3的写法最简洁，只需嵌套一层，前两种方法都必须嵌套两层
select *
  from (select row_number() over(order by t.sal) row_num, t.* from emp t) a
 where a.row_num between 1 and 3;

-- 错误方法4，这是典型的错误分页方法，数据库会先把数据select出来，这时rownum伪列根据默认顺序已经分配了数值，然后再进行order by，这样row_num顺序已经被打乱，此时的查询结果和order by前是一样的，依赖数据库默认select的结果顺序，一般数据库不保证每次select出来的顺序是一致的（当然绝大多数情况下是一致的，除非表数据做了整理）
select *
  from (select rownum row_num, t.* from emp t order by t.sal) a
 where a.row_num between 1 and 3;