DATEDIFF()函数返回两个日期之间的时间
DATEDIFF(datepart,startdate,enddate)

-- 获取两个日期之间的天数
SELECT DATEDIFF(day,'2021-01-01','2021-06-10') AS days;
SELECT DATEDIFF(day,'2021-01-01 08:00:00','2021-06-10 09:00:00') AS days;

-- 获取两个日期之间的月数
SELECT DATEDIFF(month,'2021-01-01','2021-06-10') AS months;
SELECT DATEDIFF(month,'2021-01-01 08:00:00','2021-06-10 09:00:00') AS months;

-- 获取两个日期之间的年数
SELECT DATEDIFF(year,'2021-01-01','2022-06-10') AS years;
SELECT DATEDIFF(year,'2021-01-01 08:00:00','2022-06-10 09:00:00') AS years;

-- startdate位置0表示1900-01-01，-1表示1899-12-31
SELECT DATEDIFF(day, 0, '1900-01-01');-- 结果：0
SELECT DATEDIFF(day, 0, '1900-01-31');-- 结果：30
SELECT DATEDIFF(day, -1, '1900-01-01');-- 结果：1
SELECT DATEDIFF(day, -1, '1900-01-31');-- 结果：31