SQL Server查询多对多关系

Question

我的SQL服务器中有以下多对多关系(见下图).

在大多数情况下,表tblWavelengths中有2行与表tblSensors相关,(在某些情况下只有1行,在极端情况下可能有20行)

我做了以下简单查询来从这3个表中检索数据:

select W.DateTimeID,S.SensorName,S.SensorType,W.Channel,W.PeakNr,W.Wavelength
from tblWavelengths as W
    Left Join tblSensorWavelengths as SW on W.tblWavelengthID = SW.WavelengthID
    Left Join tblSensors as S on SW.SensorID = S.SensorID
order by W.DateTimeID

运行此查询后,我得到以下结果:

这是我的问题.我想写一个查询,它只过滤那些在给定时刻(DateTimeID)在tblWavelengths表中有两行(两个不同波长)的传感器(SensorName).因此,例如,我希望得到没有77902/001传感器的结果 - 因为它在给定的时刻只有一行(一个波长)与tblSensors相关

Answer 1

您可以使用窗口函数来找出每个sensorname/datetimeid组合的波长数:

WITH Data AS
(   SELECT  W.DateTimeID,
            S.SensorName,
            S.SensorType,
            W.Channel,
            W.PeakNr,
            W.Wavelength,
            [Wcount] = COUNT(*) OVER(PARTITION BY s.SensorName, d.DateTimeID)
    from    tblWavelengths as W
            LEFT JOIN tblSensorWavelengths as SW 
                ON W.tblWavelengthID = SW.WavelengthID
            LEFT JOIN tblSensors as S 
                ON SW.SensorID = S.SensorID
)
SELECT  DateTimeID, SensorName, SensorType, Channel, PeakNr, WaveLength
FROM    Data
WHERE   Wcount = 2
ORDER BY DateTimeID;

附录

后来我意识到你可能有两个结果,一个传感器同时具有相同的波长,这将返回2个记录,但没有两个不同的波长.由于窗口函数不支持使用DISTINCT替代方法如下

WITH Data AS
(   SELECT  W.DateTimeID,
            S.SensorName,
            S.SensorType,
            W.Channel,
            W.PeakNr,
            W.Wavelength,
            W.tblWaveLengthID
    from    tblWavelengths as W
            LEFT JOIN tblSensorWavelengths as SW 
                ON W.tblWavelengthID = SW.WavelengthID
            LEFT JOIN tblSensors as S 
                ON SW.SensorID = S.SensorID

)
SELECT  d.DateTimeID, d.SensorName, d.SensorType, d.Channel, d.PeakNr, d.WaveLength
FROM    Data d
        INNER JOIN
        (   SELECT  DateTimeID, SensorName
            FROM    Data
            GROUP BY DateTimeID, SensorName
            HAVING  COUNT(DISTINCT tblWaveLengthID) = 2
        ) t
            ON t.DateTimeID = d.DateTimeID
            AND t.SensorName = d.SensorName
ORDER BY d.DateTimeID;