SQL inner join with top 1

Question

Hi I need to get table of latest known value one for each input.

I started with this

SELECT [MeterReadingId]
      ,[TimeStampUtc]
      ,[Val]
      ,[MeterReading].[InverterInputId]
      ,[Status]
  FROM [Project].[dbo].[MeterReading]
  inner join InverterInput on [MeterReading].InverterInputId = [InverterInput].InverterInputId
  inner join Inverter on [InverterInput].InverterId = [Inverter].InverterId 
  where [InverterInput].InputName = 'DayYield' and [Inverter].PlantId = 1

off course now i got all values which belong to inputs of name 'DayYield and plantId =1 My question is how can got table only of those values which have latest [TimeStampUtc]

Other words: get all those [Val] which belong to input of name 'DayYield and plantId =1 and are last inserted into table.

Answer 1

One option is a cross apply, like:

select  *
from    Devices d
cross apply
        (
        select  top 1 *
        from    Measurements m
        where   m.device_id = d.id
        order by
                m.timestamp desc
        ) m

Answer 2

For this, you can use the ranking functions, ROW_NUMBER() for example. Something like this:

WITH CTE
AS
(
    SELECT [MeterReadingId]
          ,[TimeStampUtc]
          ,[Val]
          ,m.[InverterInputId]
          ,[Status]
          ,ROW_NUMBER() OVER(PARTITION BY m.InverterInputId 
                             ORDER BY m.[TimeStampUtc] DESC) AS RN
      FROM [Project].[dbo].[MeterReading] AS m
      inner join InverterInput AS ii on m.InverterInputId = ii.InverterInputId
      inner join Inverter      AS i  on ii.InverterId     = i.InverterId 
      where ii.InputName = 'DayYield' and i.PlantId = 1
) 
SELECT *
FROM CTE
WHERE RN = 1;

Using ORDER BY m.[TimeStampUtc] DESC will give you the latest [TimeStampUtc] .

Note that: I don't know about the tables' structures you are using, but you might need to use another column to group by in the PARTITION BY clause instead of MeterReadingId.