正常使用：相当于对每个row做一次子查询

SELECT depname, empno, salary, avg(salary) OVER (PARTITION BY depname) FROM empsalary;

order by 区别window frame and partition

没有order by， window function是对整个partition起作用， partition是over语句制定的
有了order by，那window function就是对window frame起作用，window frame就是整个partition先按照order by排序，然后到当前row为止的所有数据。
There is another important concept associated with window functions: for each row, there is a set of rows within its partition called its window frame. Some window functions act only on the rows of the window frame, rather than of the whole partition. By default, if ORDER BY is supplied then the frame consists of all rows from the start of the partition up through the current row, plus any following rows that are equal to the current row according to the ORDER BY clause. When ORDER BY is omitted the default frame consists of all rows in the partition. [5] Here is an example using sum:

SELECT salary, sum(salary) OVER () FROM empsalary;
 salary |  sum  
--------+-------
   5200 | 47100
   5000 | 47100
   3500 | 47100
   4800 | 47100
   3900 | 47100
   4200 | 47100
   4500 | 47100
   4800 | 47100
   6000 | 47100
   5200 | 47100
(10 rows)

没有order by， sum的结果是一样的，因为只有一个partition。

SELECT salary, sum(salary) OVER (ORDER BY salary) FROM empsalary;
 salary |  sum  
--------+-------
   3500 |  3500
   3900 |  7400
   4200 | 11600
   4500 | 16100
   4800 | 25700
   4800 | 25700
   5000 | 30700
   5200 | 41100
   5200 | 41100
   6000 | 47100
(10 rows)

有了order by，每次只计算不断增长的window frame

可复用的window表达式

SELECT sum(salary) OVER w, avg(salary) OVER w
  FROM empsalary
  WINDOW w AS (PARTITION BY depname ORDER BY salary DESC);

window function执行顺序

window function是在where， group执行完以后才执行的，所以不可以用在where， group语句里面，只能用在select，order by子语句。
Window functions are permitted only in the SELECT list and the ORDER BY clause of the query. They are forbidden elsewhere, such as in GROUP BY, HAVING and WHERE clauses. This is because they logically execute after the processing of those clauses. Also, window functions execute after non-window aggregate functions. This means it is valid to include an aggregate function call in the arguments of a window function, but not vice versa.