MySQL的时间差函数(TIMESTAMPDIFF、DATEDIFF)、日期转换计算函数(date_add、day、date_format、str_to_date)

1. 时间差函数（TIMESTAMPDIFF、DATEDIFF）

需要用MySQL计算时间差，使用TIMESTAMPDIFF、DATEDIFF，记录一下实验结果

--0
select datediff(now(), now());

--2
select datediff('2015-04-22 23:59:00', '2015-04-20 00:00:00');

--2
select datediff('2015-04-22 00:00:00', '2015-04-20 23:59:00');

--1
select TIMESTAMPDIFF(DAY, '2015-04-20 23:59:00', '2015-04-22 00:00:00');

--2
select TIMESTAMPDIFF(DAY, '2015-04-20 00:00:00', '2015-04-22 00:00:00');

--2
select TIMESTAMPDIFF(DAY, '2015-04-20 00:00:00', '2015-04-22 12:00:00');

--2
select TIMESTAMPDIFF(DAY, '2015-04-20 00:00:00', '2015-04-22 23:59:00');

--71
select TIMESTAMPDIFF(HOUR, '2015-04-20 00:00:00', '2015-04-22 23:00:00');

--4260
select TIMESTAMPDIFF(MINUTE, '2015-04-20 00:00:00', '2015-04-22 23:00:00');

关于TIMESTAMPDIFF描述，见http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html#function_timestampdiff

如需知道TIMESTAMPDIFF还能以哪些单位返回数据，可参考TIMESTAMPADD的描述：http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html#function_timestampadd

2. 日期转换计算函数（date_add、day、date_format、str_to_date）

-- 用日期与字符串转换，计算当月第一天、下月第一天
select curdate() as '当前日期',
DATE_FORMAT(curdate(), '％Y-％m') as '当前月份',
str_to_date(concat(DATE_FORMAT(curdate(), '％Y-％m'), '-01'), '％Y-％m-％d') as '当前月的第一天',
date_add(str_to_date(concat(DATE_FORMAT(curdate(), '％Y-％m'), '-01'), '％Y-％m-％d'), interval 1 month) as '下月的第一天';

-- 当前月的最后一天
select last_day(curdate());

-- 下月第一天
select date_add(last_day(curdate()), interval 1 day);

-- 当天为当月的第几天
select day(curdate());

-- 当月第一天
select date_add(curdate(), interval 1-(day(curdate())) day);

来源：https://www.cnblogs.com/nick-huang/p/4447717.html