一. TIMESTAMPDIFF函数
语法：
TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)
参数：
unit 单位（ second 秒minute 分钟hour 小时day 天week 周month 月year 年）datetime_expr1，datetime_expr2 两个时间（后者减去前者）
示例：
SELECT TIMESTAMPDIFF(SECOND,'-06-17 00:00:00','-06-17 23:59:59'); # 结果 = 86399秒SELECT TIMESTAMPDIFF(MINUTE,'-06-17 00:00:00','-06-17 23:59:59');# 结果 = 1439分钟SELECT TIMESTAMPDIFF(HOUR,'-06-17 00:00:00','-06-17 23:59:59');# 结果 = 23小时SELECT TIMESTAMPDIFF(DAY,'-06-17 00:00:00','-06-17 23:59:59');# 结果 = 0天SELECT TIMESTAMPDIFF(WEEK,'-06-17 00:00:00','-06-17 23:59:59');# 结果 = 0周SELECT TIMESTAMPDIFF(MONTH,'-06-17 00:00:00','-06-17 23:59:59');# 结果 = 0月SELECT TIMESTAMPDIFF(YEAR,'-06-17 00:00:00','-06-17 23:59:59');# 结果 = 0年# 可以结合日期函数进行搭配使用SELECT TIMESTAMPDIFF(SECOND,'-06-17', CURRENT_DATE());SELECT TIMESTAMPDIFF(SECOND,'-06-17 00:00:00',NOW());SELECT TIMESTAMPDIFF(SECOND,'-06-17 00:00:00',CURRENT_TIMESTAMP());# 补充说明SELECT CURRENT_DATE();# 结果 = -06-17SELECT NOW();# 结果 = -06-17 23:55:55SELECT CURRENT_TIMESTAMP();# 结果 = -06-17 23:55:55
