时间函数

一、DateDiff

DateDIFF() 函数返回两个日期之间的天数

语法：

DATEDIFF(date1,date2)

例子：

SELECT DATEDIFF('2008-12-30','2008-12-29') AS DiffDate

(1)

SELECT w2.Id
FROM Weather w1, Weather w2
WHERE DATEDIFF(w2.RecordDate, w1.RecordDate) = 1
AND w1.Temperature < w2.Temperature

(2)

select activity_date as day, count(distinct user_id) as active_users  from Activity where dateDiff('2019-07-27',activity_date)<30 group by activity_date

二、指定日期的方法

看了一下评论，总结一下时间怎么指定2020年2月的方法：

(1) order_date between '2020-02-01' and '2020-02-29'  
(2) order_date like '2020-02%'  
(3) DATE_FORMAT(order_date, "%Y-%m") = "2020-02"   
(4) LEFT(order_date, 7) 或 substr(1, 7)