Question

I have a table in which i keep different meters (water meter, electricity meter) and in another table i keep the readings for each meter.
The table structure is like this :
The meter table

MeterID | MeterType | MeterName

The readings Table:

ReadingID | MeterID | Index | DateOfReading

The readings for a meter are read monthly. The thing I am trying to do now is to get the Meter information, the current reading and the previous reading in just one row. So if i would have a query, the following row would result:

MeterID | MeterType | MeterName | CurrnetIndex | LastIndex

I have the following query so far :

SELECT Meter.MeterID, Meter.MeterType, Meter.MeterName, CurrentReading.Index, PreviousReading.Index
FROM Meters AS Meter
LEFT OUTER JOIN Readings AS CurrentReading ON Meter.MeterID = CurrentReading.MeterID
LEFT OUTER JOIN Readings AS PreviousReading ON Meter.MeterID = PreviouseReading.MeterID
WHERE CurrentReading.ReadingID != PreviousReading.ReadingID AND DIMESTAMPDIFF(MONTH, CurrentReading.DateOfReading, PreviousReding.DateOfReading)=-1

The problem is that I may not have the current reading or the previous, or both, but I would still need to have the meter information retrieved. It is perfectly acceptable for me to get NULL columns, but i still need a row 🙂

Answer 1

Use:

   SELECT m.meterid,
          m.metertype,
          m.metername,
          current.index,
          previous.index
     FROM METER m
LEFT JOIN READING current ON current.meterid = m.meterid
                         AND MONTH(current.dateofreading) = MONTH(NOW())
LEFT JOIN READING previous ON previous.meterid = m.meterid
                          AND MONTH(current.dateofreading) = MONTH(NOW())-1

Being an OUTER JOIN – if the MONTH filtration is done in the WHERE clause, it can produce different results than being done in the ON clause.