SQL Count with zero values - sql

Question

I want to create a graph for my dataset for the last 24 hours.
I found a solution that works but this is pretty bad since the table I am outer joining cotains every single row in the DB since I am using the (now deprecated) "all" parameter in the group by.
Here is the solution that currently kind of works.
First I declare the date intervals that is 24 hours back in time from now. I declare it twice so I can use it later in the procedure aswell.
Declare #StartDate datetime = dateadd(hour, -24, getdate())
Declare #StartDateProc datetime = dateadd(hour, -24, getdate())
Declare #EndDate datetime = getdate()
I populate the dates into a temp table including a special formated datetsring.
create table #tempTable
(
Date datetime,
DateString varchar(11)
)
while #StartDate <= #EndDate
begin
insert into #tempTable (Date, DateString)
values (#StartDate, convert(varchar(8), #StartDate, 5) + '-' + convert(varchar(2), #StartDate, 108));
SET #StartDate = dateadd(hour,1, #StartDate);
end
This gives me data that looks like this:
Date DateString
---------------------------------------------
2015-12-09 13:59:01.970 09-12-15-13
2015-12-09 14:59:01.970 09-12-15-14
2015-12-09 15:59:01.970 09-12-15-15
2015-12-09 16:59:01.970 09-12-15-16
So what I want is to join my dataset on the matching date string and show the date even if the matching rows is zero.
Here is the rest of the query
select
Date = c.Date,
Amount = sum(c.Amount)
from
DbTable a
outer apply
(select
Date = b.DateString,
Amount = count(*)
from
#tempTable b
where
convert(varchar(8), a.DateColumn, 5) + '-' + convert(varchar(2), a.DateColumn, 108) = b.DateString
group by all
b.DateString) c
where
a.SomeParameter = 'test' and
a.DateColumn >= #StartDateProc and
a.DateColumn <= #EndDate
group by
c.Date
drop table #tempTable
Test to show actual data:
Declare #StartDate datetime = dateadd(hour, -24, getdate())
Declare #EndDate datetime = getdate()
select
dateString = convert(varchar(8),a.DateColumn,5) + '-' + convert(varchar(2),a.DateColumn, 108),
Amount = COUNT(*)
from
DbTable a
where
a.someParameter = 'test' and
a.DateColumn>= dateadd(hour, -24, getdate()) and
a.DateColumn<= getdate()
group by
convert(varchar(8),a.DateColumn,5) + '-' + convert(varchar(2),a.DateColumn, 108)
First output rows:
dateString Amount
09-12-15-14 1
09-12-15-15 1
09-12-15-16 1
09-12-15-17 3
09-12-15-18 1
09-12-15-22 3
09-12-15-23 2
As you can see here there is no data for the times from 19.00 to 21.00. This is how I want the data to be displayed:
dateString Amount
09-12-15-14 1
09-12-15-15 1
09-12-15-16 1
09-12-15-17 3
09-12-15-18 1
09-12-15-19 0
09-12-15-20 0
09-12-15-21 0
09-12-15-22 3
09-12-15-23 2

Answer 1

Normally, this would be approached with left join rather than outer apply. The logic is simple: keep all rows in the first table along with any matching information from the second. This means put the dates table first:
select tt.DateString, count(t.DateColumn) as Amount
from #tempTable tt left join
DbTable t
on convert(varchar(8), t.DateColumn, 5) + '-' + convert(varchar(2), t.DateColumn, 108) = tt.DateString and
t.SomeParameter = 'test'
where tt.Date >= #StartDateProc and
tt.Date <= #EndDate
group by tt.DateString;
In addition, your comparison for the dates seems overly complex, but if it works for you, it works.

Answer 2

The best bet here would be to use DATETIME type itself and not to lose the opportunity to use indexes:
Declare #d datetime = GETDATE()
;WITH cte1 AS(SELECT TOP 25 -1 + ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) h
FROM master..spt_values),
cte2 AS(SELECT DATEADD(hh, -h, #d) AS startdate,
DATEADD(hh, -h + 1, #d) AS enddate
FROM cte1)
SELECT c.startdate, c.enddate, count(*) as amount
FROM cte2 c
LEFT JOIN DbTable a ON a.DateColumn >= c.startdate AND
a.DateColumn < c.enddate AND
a.SomeParameter = 'test'
GROUP BY c.startdate, c.enddate