我不太喜欢数据库,实现查询时遇到以下问题。我正在使用MySql

我有一个像这样的MeteoForecast表:

CREATE TABLE MeteoForecast (
  id                   BigInt(20) NOT NULL AUTO_INCREMENT,
  localization_id      BigInt(20) NOT NULL,
  seasonal_forecast_id BigInt(20),
  meteo_warning_id     BigInt(20),
  start_date           DateTime NOT NULL,
  end_date             DateTime NOT NULL,
  min_temp             Float,
  max_temp             Float,
  icon_link            VarChar(255) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
  PRIMARY KEY (
      id
  )
) ENGINE=InnoDB AUTO_INCREMENT=3 ROW_FORMAT=DYNAMIC DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci;


它包含气象预报信息,如下所示:

id                   localization_id      start_date              end_date                min_temp             max_temp             icon_link
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1                    1                    18/09/2017 06:00:00     18/09/2017 12:00:00     15                   24                   Mostly_Cloudy_Icon.png
2                    1                    18/09/2017 12:00:00     18/09/2017 18:00:00     15                   24                   Light_Rain.png
3                    1                    19/09/2017 06:00:00     19/09/2017 12:00:00     12                   22                   Mostly_Cloudy_Icon.png
4                    1                    19/09/2017 12:00:00     19/09/2017 18:00:00     13                   16                   Mostly_Cloudy_Icon.png
5                    1                    20/09/2017 06:00:00     20/09/2017 12:00:00     18                   26                   Light_Rain.png
6                    1                    20/09/2017 12:00:00     20/09/2017 18:00:00     17                   25                   Light_Rain.png


因此,正如您在先前的数据集中看到的那样,每个记录都有一个开始日期时间和一个结束日期时间。这是因为我要在特定的一天中收集更多的预测信息(它是基于时间范围的,在示例中,每天的记录是从凌晨06:00到12:00,另一条记录是从12:00到18:00 pm) 。

因此,我创建了一个简单的查询,该查询提取了特定范围内的所有记录(在本例中为2天):

select * from MeteoForecast
where start_date between  '2017-09-18 06:00:00' and '2017-09-20 06:00:00'
order by start_date desc;


我必须以以下方式修改此查询:

对于前一个查询检索到的每个记录,都必须添加一个名为global_max_temp的新字段,该字段是同一天max_temp字段的最大值。

做一个与start_date值等于19/09/2017 ...的当天相关记录的示例...,这些是我需要获取的记录:

id                   localization_id      start_date              end_date                min_temp             max_temp             icon_link                       global_max_temp
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
3                    1                    19/09/2017 06:00:00     19/09/2017 12:00:00     12                   22                   Mostly_Cloudy_Icon.png          22
4                    1                    19/09/2017 12:00:00     19/09/2017 18:00:00     13                   16                   Mostly_Cloudy_Icon.png          22


如您所见,最后一个字段(在此模拟中手动插入)是global_max_temp,并且与该天相关的两个记录中的值都为22,因为它是与特定日期相关的所有记录中max_temp字段的最大值。

这是计算以下global_max_temp值的查询:

select max(max_temp) from MeteoForecast
where start_date = '2017-09-19 06:00:00'


如何将此功能添加到原始查询中?

最佳答案

你可以尝试这样的事情:

SELECT A.*, B.GLOBAL_MAX_TEMP
FROM (
    select id, start_date, end_date, min_temp, max_temp
    from MeteoForecast
    where start_date between  '2017-09-18 06:00:00' and '2017-09-20 06:00:00'
    ) A
INNER JOIN (SELECT  date(start_date) AS date_only, MAX(max_temp) AS GLOBAL_MAX_TEMP
            FROM MeteoForecast
            WHERE start_date BETWEEN  '2017-09-18 06:00:00' and '2017-09-20 06:00:00'
            GROUP BY date(start_date)
            ) B ON date(A.start_date) = B.date_only
ORDER by start_date desc;

关于mysql - 如何修改此查询以添加一个新字段,该字段包含原始记录总数的子集的某个字段的最大值?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/46301564/

10-16 06:54
查看更多