Question

In my table MeterReading, I have

-Id(Primary)
-ProjectMeterId
-MeterRead
-ReadDate
-ReadCount

I have multiple entries for a given ProjectMeterId. I want to update ReadCount of only those rows where ReadDate is minimum (for a given ProjectMeterId) i.e.

  Update MeterReading
  set ReadCount = 1234
  where  (ReadDate is minimun for a given ProjectMeterId)

NOTE: There are many ProjectMeterId in the table and hence multiple values have to be updated. How should I do it in a query, without declaring any SP, declaring table variables and all?? Cause doing that way, it takes more than 10 minutes to update all entries.

This is what am doing right now:

 UPDATE TTable
SET TTable.ReadCount= 222   
From
(
  Select * From MeterReading where Id in 
  (
     Select Id From
     (
        SELECT Min(Id)as Id, MIN(ReadDate) as ReadDate, ProjectMeterId  FROM MeterReading 
        WHERE ProjectMeterId IS NOT NULL AND ProjectId IS NOT NULL Group By ProjectMeterId 
     )  as temp1
 )
 ) TTable, 

(
     Select * From MeterReading where Id in
          (
        Select Id From 
         (
        SELECT MIN(ReadDate) as ReadDate, ProjectMeterId  FROM MeterReading 
        WHERE ProjectMeterId IS NOT NULL AND ProjectId IS NOT NULL Group By ProjectMeterId 

     ) as temp2
   )
) STable

Where STable.ProjectMeterId = TTable.ProjectMeterId

but Min(id) inside subQueries gives me that row which has minimum Id specific to a ProjectMeterId and not ReadDate.

What Should I Do?

Answer 1

;WITH CTE
     AS (SELECT *,
                MIN(ReadDate) OVER (PARTITION BY ProjectMeterId) AS Mn
         FROM   MeterReading)
UPDATE CTE
SET    ReadCount = 1234
WHERE  ReadDate = Mn  

Or

UPDATE m1
SET    ReadCount = 1234
FROM   MeterReading m1
WHERE  NOT EXISTS(SELECT *
                  FROM   MeterReading m2
                  WHERE  m1.ProjectMeterId = m2.ProjectMeterId
                         AND m2.ReadDate < m1.ReadDate)