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/