官网：http://dev.mysql.com/doc/refman/5.1/zh/functions.html#date-and-time-functions
秒单位计算：
select timediff('2010-04-19 14:17:00',now())+0;
结果为：50.000000
官网：http://dev.mysql.com/doc/refman/5.1/zh/functions.html#date-and-time-functions
秒单位计算：
select timediff('2010-04-19 14:17:00',now())+0;
结果为：50.000000