PostgreSQL 的 rank函数

用PostgreSQL给的例子，可以看得比较清楚：

SELECT 
depname, 
empno, 
salary, 
rank() OVER (PARTITION BY depname ORDER BY salary DESC) 
FROM empsalary;

  depname  | empno | salary | rank 
-----------+-------+--------+------
 develop   |     8 |   6000 |    1
 develop   |    10 |   5200 |    2
 develop   |    11 |   5200 |    2
 develop   |     9 |   4500 |    4
 develop   |     7 |   4200 |    5
 personnel |     2 |   3900 |    1
 personnel |     5 |   3500 |    2
 sales     |     1 |   5000 |    1
 sales     |     4 |   4800 |    2
 sales     |     3 |   4800 |    2
(10 rows)