MySQL 获取当前时间戳
1. 秒级别时间戳
自19700101 00:00:00以来按秒算，SQL如下：
mysql> select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
| 1541604376 |
+-----------------------+
1 row in set (0.00 sec)2. 当前时间戳
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2019-01-04 20:37:19 |
+---------------------+
1 row in set (0.00 sec)
