MYSQL更新最大时间戳的值

Question

我有这个问题：

  

从天气中选择* ICAO =＆＃39; SSPB＆＃39;和时间戳=   （从ICAO =＆＃39; SSPB＆＃39;）的天气中选择最大（时间戳）;

结果：

timestamp, ICAO, time, wind_angle, wind_speed, gust, visibility, condition, sky, temp, dew, pressure, humidity

2018-04-02 19:20:01, SSPB, 021919, 230, 03, , 9999, , SCT026, 23, 17, 1012, 67%

现在我想使用与select

相同的方法进行更新

  

更新天气设置wind_speed =＆＃39; 10＆＃39;其中ICAO =＆＃39; SSPB＆＃39;和   timestamp =（从天气中选择最大（时间戳）ICAO =＆＃39; SSPB＆＃39;）;

我收到此错误：

错误代码：1093。您无法指定目标表＆＃39;天气＆＃39;用于FROM子句

中的更新

Answer 1

您是衍生合并优化的受害者。但是，您可以通过创建临时表来避免使用另一个选择行。试试这个：

update weather set wind_speed = '10' where ICAO = 'SSPB' and timestamp =
( select c.cTime from 
      (select max(timestamp) as cTime 
       from weather where ICAO = 'SSPB'
       ) as c
);

Answer 2

一种可能的解决方法是对内联视图（派生表）进行JOIN。我们可以使用SELECT声明......

来实现

SELECT t.*
  FROM weather t
  JOIN ( SELECT q.ICAO
              , MAX(q.timestamp) AS max_timestamp
           FROM weather q
          WHERE q.ICAO = 'SSPB'
          GROUP BY q.ICAO
       ) s
    ON t.ICAO      = s.ICAO
   AND t.timestamp = s.max_timestamp 
 WHERE t.ICAO = 'SSPB' 

同样的模式适用于UPDATE。我们可以将SELECT ... FROM替换为UPDATE并在SET子句之前添加WHERE子句，将SELECT语句转换为UPDATE：

 UPDATE weather t
  JOIN ( SELECT q.ICAO
              , MAX(q.timestamp) AS max_timestamp
           FROM weather q
          WHERE q.ICAO = 'SSPB'
          GROUP BY q.ICAO
       ) s
    ON t.ICAO      = s.ICAO
   AND t.timestamp = s.max_timestamp 
   SET t.wind_speed = '10'
 WHERE t.ICAO = 'SSPB'