如何按日期获取滞后变量的差值？

import datetime
import mysql.connector

# 连接到MySQL数据库
cnx = mysql.connector.connect(user='username', password='password',
                              host='localhost', database='database_name')

# 获取游标
cursor = cnx.cursor()

# 获取日期范围
end_date = datetime.date.today()
start_date = end_date - datetime.timedelta(days=7)

# 获取每个日期的变量值
query = "SELECT date, variable_value FROM table_name WHERE date BETWEEN %s AND %s"
cursor.execute(query, (start_date, end_date))
results = cursor.fetchall()

# 计算滞后变量的差值
differences = []
for i in range(1, len(results)):
    difference = results[i][1] - results[i-1][1]
    differences.append(difference)

# 打印差值
for difference in differences:
    print(difference)

# 关闭游标和数据库连接
cursor.close()
cnx.close()