Select Date and Count, Group By Date -- How to show Dates with NULL Counts? - tsql

Question

SELECT
CAST(c.DT AS DATE) AS 'Date'
, COUNT(p.PatternID) AS 'Count'
FROM CalendarMain c
LEFT OUTER JOIN Pattern p
ON c.DT = p.PatternDate
INNER JOIN Result r
ON p.PatternID = r.PatternID
INNER JOIN Detail d
ON p.PatternID = d.PatternID
WHERE r.Type = 7
AND d.Panel = 501
AND CAST(c.DT AS DATE)
BETWEEN '20190101' AND '20190201'
GROUP BY CAST(c.DT AS DATE)
ORDER BY CAST(c.DT AS DATE)
The query above isn't working for me. It still skips days where the COUNT is NULL for it's c.DT.
c.DT and p.PatternDate are both time DateTime, although c.DT can't be NULL. It is actually the PK for the table. It is populated as DateTimes for every single day from 2015 to 2049, so the records for those days exist.
Another weird thing I noticed is that nothing returns at all when I join C.DT = p.PatternDate without a CAST or CONVERT to a Date style. Not sure why when they are both DateTimes.

Answer 1

There are a few things to talk about here. At this stage it's not clear what you're actually trying to count. If it's the number of "patterns" per day for the month of Jan 2019, then:
Your BETWEEN will also count any activity occurring on 1 Feb,
It looks like one pattern could have multiple results, potentially causing a miscount
It looks like one pattern could have multiple details, potentially causing a miscount
If one pattern has say 3 eligible results, and also 4 details, you'll get the cross product of them. Your count will be 12.
I'm going to assume:
you only want the distinct number of patterns, regardless of the number of details and results.
You only want January's activity
--Set up some dummy data
DROP TABLE IF EXISTS #CalendarMain
SELECT cast('20190101' as datetime) as DT
INTO #CalendarMain
UNION ALL SELECT '20190102' as DT
UNION ALL SELECT '20190103' as DT
UNION ALL SELECT '20190104' as DT
UNION ALL SELECT '20190105' as DT --etc etc
;
DROP TABLE IF EXISTS #Pattern
SELECT cast('1'as int) as PatternID
,cast('20190101 13:00' as datetime) as PatternDate
INTO #Pattern
UNION ALL SELECT 2,'20190101 14:00'
UNION ALL SELECT 3,'20190101 15:00'
UNION ALL SELECT 4,'20190104 11:00'
UNION ALL SELECT 5,'20190104 14:00'
;
DROP TABLE IF EXISTS #Result
SELECT cast(100 as int) as ResultID
,cast(1 as int) as PatternID
,cast(7 as int) as [Type]
INTO #Result
UNION ALL SELECT 101,1,7
UNION ALL SELECT 102,1,8
UNION ALL SELECT 103,1,9
UNION ALL SELECT 104,2,8
UNION ALL SELECT 105,2,7
UNION ALL SELECT 106,3,7
UNION ALL SELECT 107,3,8
UNION ALL SELECT 108,4,7
UNION ALL SELECT 109,5,7
UNION ALL SELECT 110,5,8
;
DROP TABLE IF EXISTS #Detail
SELECT cast(201 as int) as DetailID
,cast(1 as int) as PatternID
,cast(501 as int) as Panel
INTO #Detail
UNION ALL SELECT 202,1,502
UNION ALL SELECT 203,1,503
UNION ALL SELECT 204,1,502
UNION ALL SELECT 205,1,502
UNION ALL SELECT 206,1,502
UNION ALL SELECT 207,2,502
UNION ALL SELECT 208,2,503
UNION ALL SELECT 209,2,502
UNION ALL SELECT 210,4,502
UNION ALL SELECT 211,4,501
;
-- create some variables
DECLARE #start_date as date = '20190101';
DECLARE #end_date as date = '20190201'; --I assume this is an exclusive end date
SELECT cal.DT
,isnull(patterns.[count],0) as [Count]
FROM #CalendarMain cal
LEFT JOIN ( SELECT cast(p.PatternDate as date) as PatternDate
,COUNT(DISTINCT p.PatternID) as [Count]
FROM #Pattern p
JOIN #Result r ON p.PatternID = r.PatternID
JOIN #Detail d ON p.PatternID = d.PatternID
WHERE r.[Type] = 7
and d.Panel = 501
GROUP BY cast(p.PatternDate as date)
) patterns ON cal.DT = patterns.patternDate
WHERE cal.DT >= #start_date
and cal.DT < #end_date --Your code would have included 1 Feb, which I assume was unintentional.
ORDER BY cal.DT
;