Question

我有一个选择陈述

SELECT MAX(windspeed) as maxwind, count(if (avgspeed<0.8, avgspeed,0)) as no_cases 
from towerdata 
where datetime between '2013-01-01 00:00:00' and '2013-01-01 23:00:00'

我想获得windspeed等于maxwind的'datetime'值。如何构建sql？

我的目的是获得最大风速的值，以及在此速度下风吹的时间。当风速小于0.8米/秒时也没有。我可以使用两个单独的select语句，但想减少行

Answer 1

按windspeed排序并仅拍摄第一张唱片

SELECT datetime
from towerdata 
where datetime between '2013-01-01 00:00:00' and '2013-01-01 23:00:00'
order by windspeed desc
limit 1

Answer 2

这是一种方法，使用where子句中的子查询：

select max(windspeed) as max_windspeed, count(*)
from towerdata td
where datetime between '2013-01-01 00:00:00' and '2013-01-01 23:00:00' and
      windspeed = (select max(windspeed) as maxws
                   from towerdata
                   where datetime between '2013-01-01 00:00:00' and '2013-01-01 23:00:00'
                  )

编辑：

我意识到你想要一个单独的计数，所以使用一个连接：

select max(td.maxws) as max_windspeed,
       max(case when td.windspeed = td.maxws then td.datetime end) as maxws_datetime,
       sum(case when td.avgspeed < 0.8 then td.avgspeed else 0 end)
from towerdata td left outer join
     (select max(windspeed) as maxws
      from towerdata
      where datetime between '2013-01-01 00:00:00' and '2013-01-01 23:00:00'
     ) tdmax
     on td.windspeed = tdmax.windspeed
where datetime between '2013-01-01 00:00:00' and '2013-01-01 23:00:00';

顺便说一下，表达式：

count(if (avgspeed<0.8, avgspeed,0))

可能没有做你想要的。它会计算行数，与count(*)相同。 count()返回非NULL值的数量。这就是为什么我用case替换它。

MYSQL查找最大值和日期对应于同一select语句中的值

2 个答案: