MySQL and Python implement ranking window function _ MySQL

Source: Internet
Author: User
MySQL and Python implement the ranking window function python

Most databases provide window functions, such as RANK and ROW_NUMBER. MySQL is not directly provided in this respect, but it can be implemented in disguise. I have previously written the implementation of row_number. today I have time to post the implementation of rank.
Here, I use MySQL and Python to implement rank window functions respectively.
Original table information:
t_girl=# /d group_concat; Table "ytt.group_concat"Column| Type| Modifiers ----------+-----------------------+----------- rank | integer |username | character varying(20) |
Table data
t_girl=# select * from group_concat; rank | username ------+----------100 | Lucy127 | Lucy146 | Lucy137 | Lucy104 | Lucy121 | Lucy136 | Lily100 | Lily100 | Lily105 | Lily136 | Lily149 | ytt116 | ytt116 | ytt149 | ytt106 | ytt117 | ytt(17 rows)Time: 0.638 ms
PostgreSQL rank window function example:
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 provides group_concat aggregate functions for disguised implementation:
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)
Of course, if MySQL SQL is unfamiliar, it can be processed by a program. for example, if I use python to implement the rank function, the execution result is as follows: (the final part of the script source code)
>>> ================================ 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.
Attach the script code:
from __future__ import print_functionfrom datetime import date, datetime, timedeltaimport mysql.connectorimport 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 cnxdef 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 + ", " + c2rs.execute(query0,multi=False)if rs.with_rows:rows = rs.fetchall()else:return "No rows affected."i = 0j = 0k = 1result = []field1_compare = rows[0][0]field2_compare = rows[0][1]while i < len(rows):if field1_compare == rows[i][0]:j += 1if field2_compare != rows[i][1]:field2_compare =rows[i][1]k = jresult.append((rows[i][0],rows[i][1],k))else:j = 1k = 1field1_compare = rows[i][0]result.append((rows[i][0],rows[i][1],k))i += 1i = 0rows_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 += 1rs.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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