函数： DATEDIFF(datepart,startdate,enddate)
具体实例：
1 --相差年数 结果0 2 SELECT DATEDIFF(yy,'2008-12-29','2008-12-1') AS DiffDate 3 4 --相差年数 结果1 5 SELECT DATEDIFF(yy,'2008-12-29','2009-12-01') AS DiffDate
1 --相差季度 结果1 2 SELECT DATEDIFF(qq,'2008-02-02','2008-06-30') AS DiffDate 3 4 --相差季度 结果2 5 SELECT DATEDIFF(qq,'2008-02-01','2008-07-01') AS DiffDate
1 2 --相差月 结果4 3 SELECT DATEDIFF(m,'2008-02-02','2008-06-30') AS DiffDate 4 5 --相差月 结果5 6 SELECT DATEDIFF(m,'2008-02-10','2008-07-06') AS DiffDate
1 --相差天数 结果120 2 SELECT DATEDIFF(dy,'2008-03-02','2008-06-30') AS DiffDate 3 4 --相差天数 结果0 5 SELECT DATEDIFF(dy,'2008-02-10','2008-02-10') AS DiffDate
注意：我这里设置了datepart = dy 或者 dd 感觉没啥区别，路过的小伙伴求解答疑惑？
1 --相差小时 结果 20h 2 SELECT DATEDIFF(hh,'2008-03-02 01:01:01','2008-03-02 21:01:01') AS DiffDate 3 4 --相差小时 结果 20h 5 SELECT DATEDIFF(hh,'2008-03-02 01:01:01','2008-03-02 21:59:01') AS DiffDate 6 7 --相差小时 结果 1h 8 SELECT DATEDIFF(hh,'2008-03-02 01:10:01','2008-03-02 02:11:02') AS DiffDate
1 --相差秒数 结果 -10s 2 SELECT DATEDIFF(s,'2008-03-02 10:10:30','2008-03-02 10:10:20') AS DiffDate 3 4 --相差秒数 结果28800s 5 SELECT DATEDIFF(s,'2019-06-04 01:01:01','2019-06-04 09:01:01') AS DiffDate
参考资料：http://www.w3school.com.cn/sql/func_datediff.asp