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Max of top numbers in database

I have a database that contains several different columns including datatime, partnumber, metricname, and data.  I am trying to use an sqlcommand in vb.net that will give me the max(data) for just the top 30 records among many.  The where statement would include partnumber, and metricname.

How do I do this - ordering just top 30 records (desc order) and getting the max aggregate of that 'data' column?

Can I do this in one statement?

Thanks in advance for the help.
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rckrch
Asked:
rckrch
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1 Solution
 
PortletPaulCommented:
I think you can use the analytic form of MAX() here, like this
SELECT TOP 30
      datatime
    , partnumber
    , metricname
    , MAX(data) OVER (PARTITION BY (SELECT 1)) AS data
FROM your_table
WHERE partnumber = 'x'
      AND metricname = 'y'
ORDER BY
      datatime
      , partnumber

Open in new window

note the "select 1" inside the partition by is used to get the same value on each row (i.e. all rows are treated as the same "partition")
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rckrchAuthor Commented:
Thanks very much for the reply PortletPaul, but this still does not work.  I inserted the code you have above, but the it still looks at all of the data to get the MAX value instead of just the top 30.

Am I using you code correctly?  I still don't understand what 'Select 1' is and how this partition works.  Should this 'Select 1' be a column name?

This is what I put in:

SELECT TOP 30 MAX(MeanValue) OVER(PARTITION BY (Select 1)) As MaxVal
FROM DataTable
WHERE DieNumber=@DieNumber, MetricName=@MetricName
ORDER BY RecordID DESC
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PortletPaulCommented:
I thought this is what you wanted:
" it still looks at all of the data to get the MAX value instead of just the top 30."
so that is why I proposed it.

Try this:
SELECT
      datatime
    , partnumber
    , metricname
    , MAX(data) OVER (PARTITION BY (SELECT 1)) AS data
FROM (
        SELECT TOP 30
              datatime
            , partnumber
            , metricname
            , data
        FROM your_table
        WHERE partnumber = 'x'
              AND metricname = 'y'
        ORDER BY
              datatime
              , partnumber
     ) as derived

Open in new window

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rckrchAuthor Commented:
This works perfectly.  Thanks for the help.  However, I would greatly appreciate it if you could explain how this works.  

1.  What does 'SELECT 1' do?  What is it selecting?
2.  What does 'as derived' do?

Thanks,
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PortletPaulCommented:
"select 1" does exactly that, the number 1 is provided as the result.
It is used here as a "device" so that the partition by clause is valid.

I could have used any constant value
select 'x'
would have worked for example.

'as derived' is the alias I chose for the subquery, it could have been a different name e.g.

AS X

AS Y

AS abcd
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