1、使用TIMEDIFF函数，求time差值
MySQL timediff(time1,time2)：两个日期相减 time1 - time2，返回 time 差值。
mysql> SELECT TIMEDIFF('12:00','10:00') diff;
+----------+
| diff |
+----------+
| 02:00:00 |
+----------+
1 row in set (0.00 sec)
注意：timediff(time1,time2) 函数的两个参数类型必须相同。
2、使用DATEDIFF函数，求date差值
MySQL datediff(date1,date2)：两个日期相减 date1 - date2，返回天数。
mysql> select datediff('2008-08-08', '2008-08-01');
+--------------------------------------+
| datediff('2008-08-08', '2008-08-01') |
+--------------------------------------+
| 7 |
+--------------------------------------+
1 row in set (0.01 sec)
3、使用TIMESTAMPDIFF()函数
TIMESTAMPDIFF(interval,datetime_expr1,datetime_expr2)
函数共有三个参数，三个参数的取值分别是：
-
interval：比较的类型，可取值FRAC_SECOND、SECOND、 MINUTE、 HOUR、 DAY、 WEEK、 MONTH、 QUARTER 或YEAR
-
datetime_expr1和datetime_expr2是待比较的两个时间，用后面的时间减去前面的时间
mysql> SELECT TIMESTAMPDIFF(DAY,'2018-12-01 12:23:59','2018-12-03 16:29:50');
+----------------------------------------------------------------+
| TIMESTAMPDIFF(DAY,'2018-12-01 12:23:59','2018-12-03 16:29:50') |
+----------------------------------------------------------------+
| 2 |
+----------------------------------------------------------------+
1 row in set (0.00 sec)
