sql：mysql：函数：TIMESTAMPDIFF函数实现TimeStamp字段相减，求得时间差

参考：https://blog.csdn.net/yiifaa/article/details/78611079

https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_microsecond

mysql> SELECT TIMESTAMPDIFF(MONTH,‘2003-02-01‘,‘2003-05-01‘);
        -> 3
mysql> SELECT TIMESTAMPDIFF(YEAR,‘2002-05-01‘,‘2001-01-01‘);
        -> -1
mysql> SELECT TIMESTAMPDIFF(MINUTE,‘2003-02-01‘,‘2003-05-01 12:05:55‘);
        -> 128885