mysql时间差函数datediff和TIMESTAMPDIFF

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7.时间差函数：datediff(date1,date2)
（1）select datediff(now(), now());#0
（2）select datediff('2018-04-22 23:59:00', '2018-04-20 00:00:00');#2
8.时间差函数：TIMESTAMPDIFF
select TIMESTAMPDIFF(DAY, '2018-04-20 00:00:00', '2018-04-22 23:59:00');#2
select TIMESTAMPDIFF(HOUR, '2018-04-20 00:00:00', '2018-04-22 23:00:00');#71
select TIMESTAMPDIFF(MINUTE, '2018-04-20 00:00:00', '2018-04-22 23:00:00');#4260
select last_day(curdate());#返回当月最后一天
select date_add(last_day(curdate()), interval 1 day);#返回下月第一天
select day(curdate());#当天为当月的第几天
select date_add(curdate(), interval 1-(day(curdate())) day);#当月第一天