SQL LIMIT 动态 N答案

【问题标题】：SQL LIMIT dynamic NSQL LIMIT 动态 N
【发布时间】：2019-06-19 10:36:41
【问题描述】：

我想从具有指定县编号的所有站点获取（最后一行）平均空气温度。

因此，我的解决方案类似于

SELECT AVG(air_temperature) 
  FROM weather 
 WHERE station_id IN (
       SELECT station_id 
         FROM stations 
        WHERE county_number = 25
       )
 ORDER 
    BY id DESC 
 LIMIT 1;

显然，这并没有给出正确的行，因为它返回了基于曾经记录的一个站点的所有 air_temperature 的平均 air_temperature。

回到问题，我想从具有指定县编号的每个站点获取最后插入行的平均空气温度。

表格天气

+------------------+-------------+------+-----+---------+----------------+
| Field            | Type        | Null | Key | Default | Extra          |
+------------------+-------------+------+-----+---------+----------------+
| id               | int(11)     | NO   | PRI | NULL    | auto_increment |
| station_id       | char(20)    | YES  | MUL | NULL    |                |
| timestamp        | timestamp   | YES  |     | NULL    |                |
| air_temperature  | float       | YES  |     | NULL    |                |
+------------------+-------------+------+-----+---------+----------------+

台站

+---------------+-------------+------+-----+---------+-------+
| Field         | Type        | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+-------+
| station_id    | char(20)    | NO   | PRI | NULL    |       |
| county_number | int(10)     | YES  |     | NULL    |       |
+---------------+-------------+------+-----+---------+-------+

表格被最小化

【问题讨论】：

标签： mysql sql group-by aggregate-functions

【解决方案1】：

我建议使用 join 和一些过滤来执行此操作：

select avg(w.air_temperature)
from weather w join
     stations s
     on w.station_id = s.station_id
where s.county_number = 25 and
      w.timestamp = (select max(w2.timestamp) from weather w2 where w2.station_id = w.station_id)

【讨论】：

【解决方案2】：

您可以通过检查max(timestamp)获取最后插入的行：

SELECT 
  AVG(w.air_temperature) 
FROM weather w 
INNER JOIN (
  SELECT station_id, max(timestamp) maxtimestamp FROM weather GROUP BY station_id        
) t
ON w.station_id = t.station_id AND w.timestamp = t.maxtimestamp
WHERE 
  w.station_id IN (SELECT station_id FROM stations WHERE county_number = 25)

【讨论】：

【解决方案3】：

更新：我刚刚注意到您的 timestamp 列可以为空，并且您正在谈论“最后插入的行”。那是ID最大的那个。因此：

从 MySQL 8 开始，您可以使用窗口函数来读取表一次：

select avg(air_temperature) 
from
(
  select air_temperature, id, max(id) over (partition by station_id) as max_id
  from weather 
  where station_id in (select station_id from stations where county_number = 25)
) analyzed
where id = max_id;

在旧版本中，您必须阅读该表两次：

select avg(air_temperature) 
from weather 
where (station_id, id) in
(
  select station_id, max(id)
  from weather 
  where station_id in (select station_id from stations where county_number = 25)
  group by station_id
);

【讨论】：