MySQL 以及 Python 实现排名窗口函数

大部分数据库都提供了窗口函数，比如RANK,ROW_NUMBER等等。 MySQL 这方面没有直接提供，但是可以变相的实现，我以前写了row_number 的实现，今天有时间把 rank 的实现贴出来。

这里，我用MySQL 以及Python 分别实现了rank 窗口函数。
原始表信息：

t_girl=# \d group_concat;
           Table "ytt.group_concat"
  Column  |         Type          | Modifiers 
----------+-----------------------+-----------
 rank     | integer               | 
 username | character varying(20) | 

表数据

t_girl=# select * from group_concat;
 rank | username 
------+----------
  100 | Lucy
  127 | Lucy
  146 | Lucy
  137 | Lucy
  104 | Lucy
  121 | Lucy
  136 | Lily
  100 | Lily
  100 | Lily
  105 | Lily
  136 | Lily
  149 | ytt
  116 | ytt
  116 | ytt
  149 | ytt
  106 | ytt
  117 | ytt
(17 rows)


Time: 0.638 ms

PostgreSQL 的rank 窗口函数示例：

t_girl=# select username,rank,rank() over(partition by username order by rank desc) as rank_cnt from group_concat;          
 username | rank | rank_cnt 
----------+------+----------
 Lily     |  136 |        1
 Lily     |  136 |        1
 Lily     |  105 |        3
 Lily     |  100 |        4
 Lily     |  100 |        4
 Lucy     |  146 |        1
 Lucy     |  137 |        2
 Lucy     |  127 |        3
 Lucy     |  121 |        4
 Lucy     |  104 |        5
 Lucy     |  100 |        6
 ytt      |  149 |        1
 ytt      |  149 |        1
 ytt      |  117 |        3
 ytt      |  116 |        4
 ytt      |  116 |        4
 ytt      |  106 |        6
(17 rows)


Time: 131.150 ms

MySQL 提供了group_concat 聚合函数可以变相的实现：

mysql> 
select a.username, a.rank, find_in_set(a.rank,b.rank_gp) as rank_cnt 
from group_concat as a ,
(select username,group_concat(rank order by rank desc separator ‘,‘)  as rank_gp from group_concat group by username 
) b 
where a.username = b.username order by a.username asc,a.rank desc;
+----------+------+----------+
| username | rank | rank_cnt |
+----------+------+----------+
| Lily     |  136 |        1 |
| Lily     |  136 |        1 |
| Lily     |  105 |        3 |
| Lily     |  100 |        4 |
| Lily     |  100 |        4 |
| Lucy     |  146 |        1 |
| Lucy     |  137 |        2 |
| Lucy     |  127 |        3 |
| Lucy     |  121 |        4 |
| Lucy     |  104 |        5 |
| Lucy     |  100 |        6 |
| ytt      |  149 |        1 |
| ytt      |  149 |        1 |
| ytt      |  117 |        3 |
| ytt      |  116 |        4 |
| ytt      |  116 |        4 |
| ytt      |  106 |        6 |
+----------+------+----------+
17 rows in set (0.02 sec)

当然了，如果MySQL SQL不太熟悉，可以用程序来处理，比如我下面用python 实现了rank 函数，执行结果如下：（脚本源代码最后）

>>> ================================ RESTART ================================
>>> 
 username |   rank   | rank_cnt 
--------------------------------
ytt       |149       |1         
ytt       |149       |1         
ytt       |117       |3         
ytt       |116       |4         
ytt       |116       |4         
ytt       |106       |6         
Lucy      |146       |1         
Lucy      |137       |2         
Lucy      |127       |3         
Lucy      |121       |4         
Lucy      |104       |5         
Lucy      |100       |6         
Lily      |136       |1         
Lily      |136       |2         
Lily      |105       |3         
Lily      |100       |4         
Lily      |100       |4         
(17 Rows.)
Time:     0.162 Seconds.

附上脚本代码：

from __future__ import print_function
from datetime import date, datetime, timedelta
import mysql.connector
import time
# Created by ytt 2014/5/14.
# Rank function implement.
def db_connect(is_true):
    cnx = mysql.connector.connect(host=‘192.168.1.131‘,port=‘3306‘,user=‘python_user‘, password=‘python_user‘,database=‘t_girl‘,autocommit=is_true)
    return cnx
def db_rs_rank(c1 =‘username desc‘ ,c2 = ‘ rank desc‘):
    # c1: partition column.
    # c2: sort column.
    time_start = time.time()
    cnx = db_connect(True)
    rs = cnx.cursor()
    query0 = "select username,rank from group_concat order by " + c1 + ", " + c2
    rs.execute(query0,multi=False)
    if rs.with_rows:
        rows = rs.fetchall()
    else:
        return "No rows affected."
    i = 0
    j = 0
    k = 1
    result = []
    field1_compare = rows[0][0]
    field2_compare = rows[0][1]
    while i < len(rows):
        if field1_compare == rows[i][0]:
            j += 1
            if field2_compare != rows[i][1]:
                field2_compare =rows[i][1]
                k = j
            result.append((rows[i][0],rows[i][1],k))
        else:
            j = 1
            k = 1
            field1_compare = rows[i][0]
            result.append((rows[i][0],rows[i][1],k))
        i += 1
    i = 0
    rows_header = list(rs.column_names)
    rows_header.append(‘rank_cnt‘)
    print (rows_header[0].center(10,‘ ‘) + ‘|‘ + rows_header[1].center(10,‘ ‘) + ‘|‘ + rows_header[2].center(10,‘ ‘))
    print (‘-‘.center(32,‘-‘))
    while i < len(result):
        print (result[i][0].ljust(10,‘ ‘) + ‘|‘ + str(result[i][1]).ljust(10,‘ ‘) + ‘|‘ + str(result[i][2]).ljust(10,‘ ‘))
        i += 1
    rs.close()
    cnx.close()
    time_end = time.time()
    print (‘(‘ + str(len(rows))+ ‘ Rows.)‘)
    print ("Time:" + str(round((time_end-time_start),3)).rjust(10,‘ ‘) + ‘ Seconds.‘)    
if __name__==‘__main__‘:
    db_rs_rank()
    

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