mysql中的unix_timestamp函数

偶然看到MySQL的一个函数 unix_timestamp()，不明就里，于是就试验了一番。

unix_timestamp()函数的作用是返回一个确切的时间点的UNIX时间戳，这个Unix时间戳是一个无符号整数。 unix_timestamp()函数有两种重载形式，一是不带任何参数，另外一个是带有一个Date或DateTime或TimeStamp类型的参 数。

unix_timestamp()，返回自1970-1-1 8:00:00开始到当前系统时间为止的秒数。

unix_timestamp(date)，返回1970-1-1 8:00:00开始到date所代表的时间为止的秒数，对于早于1970-1-1 8:00:00的时间，总是返回 0 。

注意：因为中国是东八区，所以是8:00:00.

mysql> select unix_timestamp();
+------------------+
| unix_timestamp() |
+------------------+
|       1303195194 |
+------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp(current_timestamp());
+-------------------------------------+
| unix_timestamp(current_timestamp()) |
+-------------------------------------+
|                            1303195204 |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp('2011-4-19 12:00:00');
+--------------------------------------+
| unix_timestamp('2011-4-19 12:00:00') |
+--------------------------------------+
|                             1303185600 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp('1970-1-1 6:00:00');
+------------------------------------+
| unix_timestamp('1970-1-1 6:00:00') |
+------------------------------------+
|                                     0 |
+------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp('1970-1-1 8:00:00');
+------------------------------------+
| unix_timestamp('1970-1-1 8:00:00') |
+------------------------------------+
|                                     0 |
+------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp('1970-1-1 8:00:01');
+------------------------------------+
| unix_timestamp('1970-1-1 8:00:01') |
+------------------------------------+
|                                     1 |
+------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp('1970-1-1 8:01:00');
+------------------------------------+
| unix_timestamp('1970-1-1 8:01:00') |
+------------------------------------+
|                                 60 |
+------------------------------------+
1 row in set (0.00 sec)

了解了这个函数以后，就想如果知道了UNIX时间戳，如何换算成其对就的时间呢？于是想到了以下方法：

mysql> select date_add('1970-01-01 8:00:00',interval 1303191235 second);
+-----------------------------------------------------------+
| date_add('1970-01-01 8:00:00',interval 1303191235 second) |
+-----------------------------------------------------------+
| 2011-04-19 13:33:55                                          |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

呵呵，没有想到的是，MySQL也提供了一个函数，叫做 from_unixtime(unixtime)，这个函数和上面那个函数表达式的结果完全相同：

mysql> select from_unixtime(1303191235);
+---------------------------+
| from_unixtime(1303191235) |
+---------------------------+
| 2011-04-19 13:33:55        |
+---------------------------+
1 row in set (0.00 sec)