mysq 日期相减

mysql> desc test200;
+---------------+----------+------+-----+---------+-------+
| Field         | Type     | Null | Key | Default | Extra |
+---------------+----------+------+-----+---------+-------+
| id            | int(11)  | YES  |     | NULL    |       |
| registerTime  | datetime | YES  |     | NULL    |       |
| lastLoginTime | datetime | YES  |     | NULL    |       |
+---------------+----------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> select * from test200;
+------+---------------------+---------------------+
| id   | registerTime        | lastLoginTime       |
+------+---------------------+---------------------+
|    1 | 2016-10-21 09:06:23 | 2016-10-21 09:06:56 |
|    1 | 2016-10-21 09:16:11 | 2016-10-21 09:16:11 |
+------+---------------------+---------------------+
2 rows in set (0.00 sec)

mysql> select lastLoginTime - registerTime from test200;
+------------------------------+
| lastLoginTime - registerTime |
+------------------------------+
|                           33 |
|                            0 |
+------------------------------+
2 rows in set (0.00 sec)

mysql> select (UNIX_TIMESTAMP(lastLoginTime) - UNIX_TIMESTAMP(registerTime)) from test200;
+----------------------------------------------------------------+
| (UNIX_TIMESTAMP(lastLoginTime) - UNIX_TIMESTAMP(registerTime)) |
+----------------------------------------------------------------+
|                                                             33 |
|                                                              0 |
+----------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> select now(),lastLoginTime,(UNIX_TIMESTAMP(now()) - UNIX_TIMESTAMP(lastLoginTime)) from test200;
+---------------------+---------------------+---------------------------------------------------------+
| now()               | lastLoginTime       | (UNIX_TIMESTAMP(now()) - UNIX_TIMESTAMP(lastLoginTime)) |
+---------------------+---------------------+---------------------------------------------------------+
| 2016-10-21 09:32:32 | 2016-10-21 09:06:56 |                                                    1536 |
| 2016-10-21 09:32:32 | 2016-10-21 09:16:11 |                                                     981 |
+---------------------+---------------------+---------------------------------------------------------+
2 rows in set (0.01 sec)