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来自https://blog.csdn.net/sinat_36190649/article/details/107941638，根据需求略做修改

import pymysql
import pandas as pd
import gc
import time
import threading
 
class Sql_df(object):
    def __init__(self,input_db):
        self.host = ip地址
        self.port = 3306
        self.username = 用户名
        self.password = 密码
        self.input_db = input_db
        self.conn = pymysql.connect(host = self.host,port = self.port,user = self.username,passwd = self.password,db = self.input_db,charset = 'utf8')
    def sql_input_all(self,sql_state):  #全部读取用pd处理
        cur_1 = self.conn.cursor(cursor = pymysql.cursors.DictCursor)
        cur_1.execute(sql_state+' limit 1')
        column_df = cur_1.fetchall()
        column_list = column_df[0].keys()
        cur_2 = self.conn.cursor()
        start_time = time.time()
        cur_2.execute(sql_state)
        tmp_list = cur_2.fetchall()
        result_df = pd.DataFrame(list(tmp_list),columns = column_list)
        end_time = time.time()
        during_time = round(end_time-start_time,0)/60
        print('input data has spend %f minutes'%during_time)
        return result_df
    def sql_input_batch(self,sql_state,nums_sql_state,batch_size):    #分批处理写入txt文件
        cur_2 = self.conn.cursor()
        start_time = time.time()
        cur_2.execute(nums_sql_state)
        nums_sample = cur_2.fetchall()[0][0]
        batches = nums_sample//batch_size
        cur_3 = self.conn.cursor()
        for i in range(batches):
            cur_3.execute(sql_state+' limit '+str(i*batch_size)+','+str(batch_size))
            tmp_d = cur_3.fetchall()
            tmp_list = ['. '.join(i) for i in tmp_d]
            with open('./all_data.txt', 'a+', encoding='utf-8') as f:
                for i in tmp_list:
                    line = i.split()
                    if len(line) < 30:
                        continue
                    
                    new = content_pro(line)
                    f.write(new+'\n')
            del tmp_d
            del tmp_list
            gc.collect()
            gc.collect()
        last_index = batches*batch_size
        cur_3.execute(sql_state+' limit '+str(last_index)+','+str(nums_sample-last_index))
        tmp_d = cur_3.fetchall()
        tmp_list = ['. '.join(i) for i in tmp_d]
        with open('./all_data.txt', 'a+', encoding='utf-8') as f:
            for i in tmp_list:
                line = i.split()
                if len(line) < 30:
                        continue
                    
                new = content_pro(line)
                f.write(new+'\n')
        end_time = time.time()
        during_time = round(end_time-start_time,0)/60
        print('input data has spend %f minutes'%during_time)
        del tmp_d
        gc.collect()
        return 'success'


if __name__ == '__main__':
    data_input = Sql_df(数据库名)
    rr = data_input.sql_input_batch('select * from 表名','select count(1) from 表名',1000000)        

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