Mysql 字符串、时间、时间戳相互转换，相减获取秒数

涉及的函数
date_format(date, format) 函数
unix_timestamp() 函数
str_to_date(str, format) 函数
from_unixtime(unix_timestamp, format) 函数

1.字符串转化成时间
select
str_to_date('2019-09-12 00:16:50','%Y-%m-%d %H:%i:%s'),
str_to_date('2019-09-12 00:16:50','%Y-%m-%d %H')

2019-09-12 00:16:50    2019-09-12 00:00:00

2.字符串转化成时间戳
select
unix_timestamp('2019-09-12 00:00:50'),
unix_timestamp('2019-09-12 00:01:00');

1568217650    1568217660

3.时间转换成字符串
select
date_format(now(), '%Y-%m-%d'),
date_format(now(), '%Y-%m-%d %H:%i:%s')

2020-08-24    2020-08-24 14:51:26

4.时间转换成时间戳
select
unix_timestamp(now());

1598251906

5.时间戳转换成时间
select from_unixtime(1598250986);  

2020-08-24 14:36:26

6.时间戳转成成字符串
select from_unixtime(1598250986,'%Y-%m-%d');

2020-08-24

要想获得正确的时间相减秒数，有以下三张方式：
1、time_to_sec(timediff(t2, t1))
2、timestampdiff(second, t1, t2)
3、unix_timestamp(t2) -unix_timestamp(t1)


列子：
 select  time_to_sec(timediff(str_to_date('2019-09-12 00:17:50','%Y-%m-%d %H:%i:%s'), str_to_date('2019-09-12 00:16:50','%Y-%m-%d %H:%i:%s')))

60

 select  timestampdiff(second, str_to_date('2019-09-12 00:17:50','%Y-%m-%d %H:%i:%s'), str_to_date('2019-09-12 00:16:50','%Y-%m-%d %H:%i:%s'))

-60

 select unix_timestamp(str_to_date('2019-09-12 00:17:50','%Y-%m-%d %H:%i:%s')) -unix_timestamp(str_to_date('2019-09-12 00:16:50','%Y-%m-%d %H:%i:%s'))

60