OLAP函数语法四个部分:
1、function本身 用于对窗口中的数据进行操作；
2、partitioning clause 用于将结果集分区；
3、order by clause 用于对分区中的数据进行排序；
4、windowing clause 用于定义function在其上操作的行的集合，即function所影响的范围。

一、order by对窗口的影响
不含order by的：
SQL> select deptno,sal,sum(sal) over()
  2  from emp;
不含order by时，默认的窗口是从结果集的第一行直到末尾。
含order by的：
SQL> select deptno,sal,
  2         sum(sal) over(order by deptno) as sumsal
  3  from emp;
当含有order by时，默认的窗口是从第一行直到当前分组的最后一行。

二、用于陈列的函数
SQL> select empno, deptno, sal,
  2         rank() over
  3         (partition by deptno order by sal desc nulls last) as rank,
  4         dense_rank() over
  5         (partition by deptno order by sal desc nulls last) as dense_rank,
  6         row_number() over
  7         (partition by deptno order by sal desc nulls last) as row_number
  8   from  emp;

三、用于合计的函数

SQL> select deptno,sal,
  2         sum(sal) over (partition by deptno) as sumsal,
  3         avg(sal) over (partition by deptno) as avgsal,
  4         count(*) over (partition by deptno) as count,
  5         max(sal) over (partition by deptno) as maxsal
  6    from emp;

四、开窗语句

1、rows窗口: "rows 5 preceding"

    适用于任何类型而且可以order by多列。

   SQL> select deptno,ename,sal,
          2         sum(sal) over (order by deptno rows 2 preceding) sumsal
          3    from emp;

rows 2 preceding:将当前行和它前面的两行划为一个窗口，因此sum函数就作  用在这三行上面
SQL> select deptno,ename,sal,
  2         sum(sal) over
  3         (partition by deptno order by ename rows 2 preceding) sumsal
  4    from emp
  5  order by deptno,ename;
加了partiton by分区后之后，rows 2 preceding(窗口)只在当前分区内生效，不会影响分区之外的行。
SQL> select ename,sal,hiredate,
  2         first_value(ename)over
  3         (order by hiredate asc rows 5 preceding) first_ename,
  4         first_value(hiredate) over
  5         (order by hiredate asc rows 5 preceding) first_hiredate
  6    from emp
  7  order by hiredate asc;
order by hiredate asc rows 5 preceding: order by之后，取当前行的前5行+当前行作为窗口(共6行)。
2、"range unbounded preceding"

range unbounded preceding会把当前行之前的所有行都包含进来，但当partition by时:

SQL> select deptno,ename,sal,
  2         sum(sal) over
  3         (partition by deptno order by deptno range unbounded preceding) sumsal
  4    from emp;
SQL> select deptno,en