sql server how to find the max of a count row - sql-server-2008

Question

I have written the following query:
DECLARE #countryCode TINYINT
SET #countryCode = 1;
SELECT
DivingSite.SiteCode,
DATEPART(Month, divingDate) AS 'month number' ,
COUNT (divingNo) AS 'number of dives in month'
FROM
DivingSite
INNER JOIN
tblDiving ON DivingSite.SiteCode = tblDiving.SiteCode
WHERE
DivingSite.countryCode = #countryCode
AND divingDate > DATEADD(year, -1, GETDATE())
GROUP BY
DivingSite.SiteCode, DATEPART(Month, divingDate)
The result of this query is:
siteCode | month number | number of dives in month
--------------------------------------------------
107 1 1
108 7 2
107 8 2
The thing is - for every siteCode I want to display only the month with the biggest number of dives. So I would like to have the result:
siteCode | month number | number of dives in month
--------------------------------------------------
108 7 2
107 8 2
How do I do that?

Answer 1

Try this - use a CTE (Common Table Expression) with a ROW_NUMBER function that "partitions" your data by the siteCode and numbers each row for a single siteCode starting at one - with the one with the highest number of dives having RowNum = 1 and any others having higher row numbers.
By selecting only those rows with RowNum = 1 you get only those entries for each siteCode with the highest number of dives.
DECLARE #countryCode TINYINT
SET #countryCode = 1;
;WITH RawData AS
(
SELECT
ds.SiteCode,
MonthNumber = DATEPART(Month, divingDate),
NumberOfDives = COUNT (divingNo),
RowNum = ROW_NUMBER() OVER (PARTITION BY ds.SiteCode ORDER BY COUNT(divingNo) DESC)
FROM
dbo.DivingSite ds
INNER JOIN
dbo.tblDiving d ON ds.SiteCode = d.SiteCode
WHERE
ds.countryCode = #countryCode
AND divingDate > DATEADD(year, -1, GETDATE())
GROUP BY
ds.SiteCode, DATEPART(Month, divingDate)
)
SELECT
SiteCode,
MonthNumber,
NumberOfDives
FROM
RawData
WHERE
RowNum = 1

Answer 2

You could create a query on top of the one you already have, like so:
select
DivingSite.SiteCode,
DATEPART(Month, divingDate) as 'month number' ,
count (divingNo) as 'number of dives in month'
into #dives
from DivingSite
inner join tblDiving on DivingSite.SiteCode = tblDiving.SiteCode
where
DivingSite.countryCode = #countryCode
and divingDate > DATEADD(year,-1,GETDATE())
group by
DivingSite.SiteCode,
DATEPART(Month, divingDate)
select d.* from #dives d join
(
SELECT
SiteCode,
MAX([number of dives in month]) MaxDives
FROM #dives
GROUP BY SiteCode
) max_dives on max_dives.SiteCode = d.SiteCode
and d.[number of dives in month] = max_dives.MaxDives