Question

我正在编写SQL查询，我希望在datagrid视图中使用相同的列两次。

这是我的问题：

SELECT ElectricityMachinePanelDetails.MachineDescription AS "Machine Name",
       ReadingValue AS "Last Day Meter Reading" 
FROM ElectricityDailyMeterReadingDetails
INNER JOIN ElectricityMachinePanelDetails ON ElectricityMachinePanelDetails.MachinePanelID = ElectricityDailyMeterReadingDetails.MachinePanelID
INNER JOIN ReadingTypesDetails ON ElectricityDailyMeterReadingDetails.ReadingTypeID = ReadingTypesDetails.ReadingTypeID 
WHERE ReadingCategoryID = 'RC001' AND 
      ReadingTypesDetails.ReadingTypeID = 'RT001'

如何在数据网格中使用列名ReadingValue两次。

Answer 1

首先，在连接中使用别名而不是表名。所以你的查询可以更具可读性。

select
    empd.MachineDescription as "Machine Name",
    rtd.ReadingValue AS "Last Day Meter Reading"
from ElectricityDailyMeterReadingDetails as edmrd
    inner join ElectricityMachinePanelDetails as empd on empd.MachinePanelID = edmrd.MachinePanelID
    inner join ReadingTypesDetails as rtd on rtd.ReadingTypeID  = edmrd.ReadingTypeID
where
    rtd.ReadingCategoryID = 'RC001' and
    rtd.ReadingTypesDetails.ReadingTypeID = 'RT001'

如果您想两次加入ReadingTypesDetails，您可以轻松地执行此操作：

select
    empd.MachineDescription as "Machine Name",
    rtd.ReadingValue as "Last Day Meter Reading",
    rtd2.ReadingValue as "Another Reading"
from ElectricityDailyMeterReadingDetails as edmrd
    inner join ElectricityMachinePanelDetails as empd on empd.MachinePanelID = edmrd.MachinePanelID
    inner join ReadingTypesDetails as rtd on rtd.ReadingTypeID  = edmrd.ReadingTypeID
    inner join ReadingTypesDetails as rtd2 on rtd2.ReadingTypeID  = edmrd.ReadingTypeID
where
    rtd.ReadingCategoryID = 'RC001' and
    rtd.ReadingTypesDetails.ReadingTypeID = 'RT001' and
    --rtd2.ReadingCategoryID = '????' and
    --rtd2.ReadingTypesDetails.ReadingTypeID = '????' and

Answer 2

如果datagridview允许doublename我不确定，你可以使用它两次。否则只需给它另一个名字“Last Day Meter Reading Copy”。

Select 
ElectricityMachinePanelDetails.MachineDescription AS "Machine Name",
ReadingValue AS "Last Day Meter Reading",
ReadingValue AS "Last Day Meter Copy"
from ElectricityDailyMeterReadingDetails
INNER JOIN ElectricityMachinePanelDetails on ElectricityMachinePanelDetails.MachinePanelID = ElectricityDailyMeterReadingDetails.MachinePanelID
INNER JOIN ReadingTypesDetails ON ElectricityDailyMeterReadingDetails.ReadingTypeID = ReadingTypesDetails.ReadingTypeID where ReadingCategoryID = 'RC001' AND ReadingTypesDetails.ReadingTypeID = 'RT001'

在MSSQL中，我认为你不能拥有两次相同的列名。但在MySQL中很好。

在SQL查询中使用相同的列两次

2 个答案: