Select last value in a month for all given IDs

Question

I have 2 tables, one containing meter IDs, and another containing measurements for some of the meters in the first table. This is the table structure:

MeterConfig:

• MeterID (int)
• MeterNumber (char[16])
• Type (char[25])

Readings:

• MeterID (int)
• Date (datetime)
• Value (numeric(18,6))

I need to get the last reading (and its date) from a given period for each meter, as well as the meter number. I managed to do this in T-SQL, although I'm not particularly pleased with the way I did it using this query:

select distinct
 cfg.MeterNumber,
 (select top 1 r.Date from Readings as r where r.Date between @startdate and @endDate and r.MeterID = cfg.MeterID order by r.Date desc) as Date,
 (select top 1 r.Value from Readings as r where r.Date between @startdate and @endDate and r.MeterID = cfg.MeterID order by r.Date desc) as Value
from
 MeterConfig cfg, Readings r1
where cfg.MeterID = r1.MeterID and r1.Date between @startdate and @endDate;

How can I do this more efficiently?

Answer 1

Assuming the dates in Readings are unique (ic include a timestamp), following should be equivalent to your query.

SELECT  DISTINCT cfg.MeterNumber
        , r1.Date
        , r1.Value         
FROM    MeterConfig cfg
        INNER JOIN Readings r1 ON cfg.MeterID = r1.MeterID 
        INNER JOIN (        
          SELECT  date = MAX(r.Date)
          FROM    Readings r
          WHERE   r.Date BETWEEN @StartDate AND @EndDate
        ) r2 On r2.date = r1.date          

Answer 2

WITH CTE AS (
SELECT mc.MeterID, Date, Value, ROW_NUMBER() OVER (PARTITION BY mc.MeterID ORDER BY Date DESC) as Rank
FROM  MeterConfig mc
INNER JOIN Readings rd
ON mc.MeterID = rd.MeterID
WHERE rd.Date BETWEEN @startdate AND @endDate)
SELECT * FROM CTE WHERE Rank = 1