DATENAME causes 'Distinct' to be ignored - sql-server

Question

Adding DATENAME() function to query causes duplicate rows despite 'distinct'.
TREE - TreeId, CityId, DatePlanted
WATER - WaterId, TreeId(fk), DateWatered
Table1 is one to many with Table 2
Each row in the TREE table indicates the planting of a tree. WATER table is a single instance of watering that tree. A tree is watered many times a year. You get the idea.
I need to return a report the shows the number of trees planted, by month and the number times it was watered.
SELECT t.CityId
, COUNT(distinct t.TreeId) as 'Trees Planted'
, COUNT(w.TreeId) as 'Trees Watered'
FROM TREE t
JOIN WATER w ON t.TreeId = w.TreeId
WHERE w.DateWatered between #Start AND #End
GROUP BY t.CityId
This works fine. However when I try to group by the month, the t.Treeid is no longer distinct, so the number of trees is too high.
SELECT t.CityId
, DATENAME(month, w.DateWatered)
, COUNT(distinct t.TreeId) as 'Trees Planted'
, COUNT(w.TreeId) as 'Trees Watered'
FROM TREE t
JOIN WATER w ON t.TreeId = w.TreeId
WHERE w.DateWatered between #Start AND #End
GROUP BY t.CityId, DATENAME(month, w.DateWatered)
EDIT: I have found why I am getting duplicates but not how to fix it. If a tree is watered in April 2016 then again in May 2016, I get a count of 2 trees planted and 2 trees watered where it should be one tree planted and 2 waterings. If I do the first query with no date returned, I get the correct number. So by adding the date, and even if I group by Year, then Month, with two waterings of the same tree, it is also showing the tree planted twice. I am currently investigating the use of CTEs to maybe keep each part of the query separate.

Answer 1

This demonstrates how to break the problem down into steps within a common table expression (CTE). Note that you can swap out the final select with one of the commented selects to view the intermediate results. It's a convenient way to test, debug, or understand what is going on.
One of the problems you're fighting is trying to summarize the data based only on the dates of waterings. If a tree is planted in a month that has no waterings then it wasn't being counted. The code below summarizes the plantings and waterings separately for the range of dates, then combines them into a single result set.
-- Sample data.
declare #Trees as Table ( TreeId Int Identity, CityId Int, DatePlanted Date );
declare #Waterings as Table ( WateringId Int Identity, TreeId Int, DateWatered Date );
insert into #Trees ( CityId, DatePlanted ) values
( 1, '20160115' ), ( 1, '20160118' ),
( 1, '20160308' ), ( 1, '20160318' ), ( 1, '20160118' ),
( 1, '20170105' ),
( 1, '20170205' ),
( 1, '20170401' ),
( 2, '20160113' ), ( 2, '20160130' ),
( 2, '20170226' ), ( 2, '20170227' ), ( 2, '20170228' );
insert into #Waterings ( TreeId, DateWatered ) values
( 1, '20160122' ), ( 1, '20160129' ), ( 1, '20160210' ), ( 1, '20160601' ),
( 5, '20160120' ), ( 5, '20160127' ), ( 5, '20160215' ), ( 5, '20160301' ), ( 5, '20160515' );
select * from #Trees;
select * from #Waterings;
-- Combine the data.
declare #StartDate as Date = '20100101', #EndDate as Date = '20200101';
with
-- Each tree with the year and month it was planted.
TreesPlanted as (
select CityId, TreeId,
DatePart( year, DatePlanted ) as YearPlanted,
DatePart( month, DatePlanted ) as MonthPlanted
from #Trees
where #StartDate <= DatePlanted and DatePlanted <= #EndDate ),
-- Tree plantings summarized by city, year and month.
TreesPlantedSummary as (
select CityId, YearPlanted, MonthPlanted, Count( TreeId ) as Trees
from TreesPlanted
group by CityId, YearPlanted, MonthPlanted ),
-- Each watering and the year and month it occurred.
TreesWatered as (
select CityId, W.TreeId,
DatePart( year, W.DateWatered ) as YearWatered,
DatePart( month, W.DateWatered ) as MonthWatered
from #Trees as T left outer join
#Waterings as W on W.TreeId = T.TreeId
where #StartDate <= W.DateWatered and W.DateWatered <= #EndDate ),
-- Waterings summarized by city, year and month.
TreesWateredSummary as (
select CityId, YearWatered, MonthWatered,
Count( distinct TreeId ) as Trees, Count( TreeId ) as Waterings
from TreesWatered
group by CityId, YearWatered, MonthWatered )
-- Combine the plantings and waterings for the specified period.
select Coalesce( TPS.CityId, TWS.CityId ) as CityId,
Coalesce( TPS.YearPlanted, TWS.YearWatered ) as Year,
Coalesce( TPS.MonthPlanted, TWS.MonthWatered ) as Month,
Coalesce( TPS.Trees, 0 ) as TreesPlanted,
Coalesce( TWS.Trees, 0 ) as TreesWatered,
Coalesce( TWS.Waterings, 0 ) as Waterings
from TreesPlantedSummary as TPS full outer join
TreesWateredSummary as TWS on TWS.CityId = TPS.CityId and
TWS.YearWatered = TPS.YearPlanted and TWS.MonthWatered = TPS.MonthPlanted
order by CityId, Year, Month;
-- Alternative queries for testing/debugging/understanding:
-- select * from TreesPlantedSummary order by CityId, YearPlanted, MonthPlanted;
-- select * from TreesWateredSummary order by CityId, YearWatered, MonthWatered;
And now you want the missing months (with no activity) included in the results, eh?

Answer 2

SELECT t.CityId
, ISNULL(DATENAME(month, w.DateWatered), DATENAME(month, t.DatePlanted))
, (SELECT COUNT(tDistinct.TreeId) FROM TREE tDistinct
WHERE tDistinct.TreeId = t.TreeId AND DATENAME(month, tDistinct.DatePlanted) = DATENAME(month, t.DateWatered) AND t.DatePlanted between #Start AND #End) as 'Trees Planted'
, COUNT(w.TreeId) as 'Trees Watered'
FROM TREE t
JOIN WATER w ON t.TreeId = w.TreeId
WHERE w.DateWatered between #Start AND #End
GROUP BY t.CityId, DATENAME(month, w.DateWatered), DATENAME(month, t.DatePlanted)
The only drawback here is a scenario in which no tree was watered in a month where a tree was planted your date will be null so i added a check for that...not sure what your data looks like so it may make sense to ignore the ISNULL check in favor of your original grouping
EDITED:
Based upon your requirements I do not believe CTE is necessary; based upon the additional information you have provided I have altered the query slightly to suit your needs:
`SELECT DATENAME(MONTH, myConsolidatedTree.DateAction) as myDate
,(SELECT COUNT(*)
FROM TREE AS t
WHERE
DATENAME(MONTH, myConsolidatedTree.DateAction) = DATENAME(MONTH, t.DatePlanted)
) as myNumberOfPlanted
,(SELECT COUNT(*)
FROM WATER AS w
WHERE
DATENAME(MONTH, myConsolidatedTree.DateAction) = DATENAME(MONTH, w.DateWatered)
) as myNumberOfWatered
FROM(
SELECT t.DatePlanted as DateAction
,t.TreeId as IdAction
,'PLANTED' as TreeAction
FROM TREE t
UNION
SELECT w.DateWatered as DateAction
,w.TreeId as IdAction
,'WATERED' as TreeAction
FROM WATER w) as myConsolidatedTree
WHERE myConsolidatedTree.DateAction between #StartDate and #EndDate
GROUP BY DATENAME(MONTH, myConsolidatedTree.DateAction), DATEPART(MONTH, myConsolidatedTree.DateAction)
ORDER BY DATEPART(MONTH, myConsolidatedTree.DateAction)`
While the consolidated subquery contains more information than is required for this question I left the additional TreeId and derived TreeAction columns there in case you may encounter a need for this in the future.