SQL Server查询多对多关系 [英] SQL Server Query for Many to Many Relationship

问题描述

我的SQL Server中具有以下多对多关系(请参见下图).

I have the following Many to many relationship (See the picture below) in my SQL server.

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)

In most cases there's are 2 rows in table tblWavelengths related to the table tblSensors, (in some cases only 1, and in extreme cases there can be 20 rows)

我进行了以下简单查询，以从这3个表中检索数据:

I made the following simple query to retrieve the data from those 3 tables :

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:

After running this query I got the following results :

这是我的问题.我想编写一个查询，该查询仅过滤那些在给定时间点(DateTimeID)在tblWavelengths表中具有两行(两种不同波长)的传感器(SensorName).因此，例如，我希望得到的结果没有 77902/001传感器-因为在给定的时间点只有一排(一个波长)与tblSensors相关

Here comes my problem. I want to write a query which filters only those Sensors (SensorName) which at a given moment in time (DateTimeID) has two rows (two different wavelengths) in the tblWavelengths table. So for example I want to have the results without the 77902/001 Sensor - because it has only one row (one Wavelength) related to the tblSensors at a given moment in time

推荐答案

您可以使用窗口化函数找出每种传感器名称/日期时间组合的波长数:

You could use a windowed function to find out the number of wavelengths for each sensorname/datetimeid combination:

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;

添加

经过一番思考，我意识到您可能同时为一个传感器使用相同的波长获得两个结果，这将返回2条记录，但是没有两个不同的波长.由于窗口函数不支持DISTINCT的使用，因此下面提供了一种替代方法

As an after thought I realised that you might have two results for one sensor at the same time with the same wavelength, which would return 2 records, but not have two different wavelengths. Since windowed functions don't support the use of DISTINCT an alternative is below

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;

这篇关于SQL Server查询多对多关系的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！