Python-MySQL抽取表中每个分组的前N个数据

在超大规模数据处理时，有时候需要按分组取出一部分数据，作为整体数据的参考。
例如，有表table_1，按column_1可以划分为200组，现在我们希望从这200组中每组各取出10条记录作为代表。
我们可以通过以下Python语句操作MySQL，来实现此功能：

import mysql.connector
conn = mysql.connector.connect(host = '127.0.0.1', port = 3306, user = 'root', password = 'xxx', db = 'xxx' )
cursor = conn.cursor()
for i in range(0,200): #注意limit i,j 时 i是从0开始索引的
    sql = "INSERT INTO Sample_table_1 SELECT t.column_1, t.column_2 FROM table_1 t, (SELECT * FROM table_1 group by column1 limit %s,1) p where t.column1=p.column1 limit 10;"%i
    cursor.execute(sql)
    conn.commit()
conn.close()

 当然，由于这是个插入操作，我们也可以使用excutemany来实现：

import mysql.connector
conn = mysql.connector.connect(host = '127.0.0.1', port = 3306, user = 'root', password = 'xxx', db = 'xxx' )
cursor = conn.cursor()
cursor.executemany("INSERT INTO Sample_table_1 SELECT t.column_1, t.column_2 FROM table_1 t, (SELECT * FROM table_1 group by column1 limit %s,1) p where t.column1=p.column1 limit 10;", [(i,) for i in range(0,200)] ) #注意必须保留(i,)中的","，否则会报错
conn.commit()
conn.close()

 再或者，也可以使用MySQL存储过程。

import mysql.connector
conn = mysql.connector.connect(host = '127.0.0.1', port = 3306, user = 'root', password = 'xxx', db = 'xxx' )
cursor = conn.cursor()
sql1="drop procedure if exists test;"
sql2="create procedure test() \
    begin\
    declare i bigint default 0; \
    DECLARE var BIGINT DEFAULT 0; \
    while i < 200 do \
    INSERT INTO Sample_table_1 SELECT t.column_1, t.column_2 FROM table_1 t, (SELECT * FROM table_1 group by column1 limit i,1) p where t.column1=p.column1 limit 10;\
    set i = i + 1;\
    end while;\
    end;"
sql3="call test;"
cursor.execute(sql1)
cursor.execute(sql2)
cursor.execute(sql3)
conn.commit()
conn.close()

 就执行效率来说，excutemany较excute略高一些，存储过程的效率最低。