Question

I have one table with columns channel, value and timestamp, and another table with 7 other columns with various data.

I’m joining these two together, and I want to select the maximum value of the value column within an hour, and the timestamp of the corresponding row. This is what I’ve tried, but it (obviously) doesn’t work.

SELECT
    v.channel,
    MAX(v.value),
    v.timestamp,
    i.stuff,
    ...
FROM
    Values v
INNER JOIN
    @Information i
ON i.type = v.type
GROUP BY channel, DATEPART(HOUR, timestamp), i.stuff, ...

I’m (not very surprisingly) getting the following error:

“dbo.Values.timestamp” is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause

How should I do this correctly?

Answer 1

You could use the RANK() or DENSE_RANK() features to get the results as appropriate. Something like:

;WITH RankedResults AS
(
    SELECT
        channel,
        value,
        timestamp,
        type,
        RANK() OVER (PARTITION BY DATEPART(hour,timestamp) ORDER BY value desc) as Position
    FROM
        Values
)
SELECT
   v.channel,
   v.value,
   v.timestamp,
   i.stuff
   /* other columns */
FROM
   RankedResults v
       inner join
   @Information i
       on
           v.type = i.type
WHERE
   v.Position = 1

(whether to use RANK or DENSE_RANK depends on what you want to do in the case of ties, really)

(Edited the SQL to include the join, in response to Tomas’ comment)