MySQL []> 显示阅读中的列；
| Field | Type | Null | Key | Default | Extra |
| cont | int(4) | NO | PRI | NULL | auto_increment |
| id | int(4) | YES | MUL | NULL | |
| sensor_num | int(4) | YES | | NULL | |
| value | float(5,2) | YES | | NULL | |
| time | datetime | YES | | NULL | |
从阅读中选择*；
18570 | 1 | 1 | 23.93 | 2017-08-14 12:45:01 |
| 18571 | 1 | 1 | 23.93 | 2017-08-14 12:46:01 |
| 18572 | 1 | 1 | 23.93 | 2017-08-14 12:47:01 |
| 18573 | 1 | 1 | 23.93 | 2017-08-14 12:48:01 |
| 18574 | 1 | 1 | 23.93 | 2017-08-14 12:49:02 |
| 18575 | 1 | 1 | 23.93 | 2017-08-14 12:50:02 |
| 18576 | 1 | 1 | 23.93 | 2017-08-14 12:51:02 |
| 18577 | 1 | 1 | 23.93 | 2017-08-14 12:52:02 |
| 18578 | 1 | 1 | 23.93 | 2017-08-14 12:53:02 |
| 18579 | 1 | 1 | 23.93 | 2017-08-14 12:54:02 |
| 18580 | 1 | 1 | 23.93 | 2017-08-14 12:55:02 |
| 18581 | 1 | 1 | 23.93 | 2017-08-14 12:56:02 |
| 18582 | 1 | 1 | 23.93 | 2017-08-14 12:57:02 |
| 18689 | 1 | 1 | 25.88 | 2017-08-14 14:54:56 |
| 18690 | 1 | 1 | 25.88 | 2017-08-14 14:55:56 |
| 18691 | 1 | 1 | 25.88 | 2017-08-14 14:56:56 |
| 18692 | 1 | 1 | 25.88 | 2017-08-14 14:57:56 |
| 18693 | 1 | 1 | 25.88 | 2017-08-14 14:58:56 |
| 18694 | 1 | 1 | 25.88 | 2017-08-14 14:59:56 |
| 18695 | 1 | 1 | 25.88 | 2017-08-14 15:00:56 |
| 18696 | 1 | 1 | 25.88 | 2017-08-14 15:01:56 |
| 18697 | 1 | 1 | 25.88 | 2017-08-14 15:02:56 |
| 18698 | 1 | 1 | 25.88 | 2017-08-14 15:03:56 |
| 18699 | 1 | 1 | 25.88 | 2017-08-14 15:04:56 |
| 18700 | 1 | 1 | 25.88 | 2017-08-14 15:05:56 |
如果您想要每小时的第一行，您可以按小时选择与 min(time) 组匹配的行：
select *
from reading a
inner join (
select date(time) t_date, min(time) t_time
from reading
group by date(time), hour(time)
) t on t.t_time = a.time
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