SQL Server Query for Many to Many Relationship - sql

Question

I have the following Many to many relationship (See the picture below) in my SQL server.
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)
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
After running this query I got the following results :
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

Answer 1

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;
ADDENDUM
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;