Using an Aggregate function with a sub query - sql

Question

We get a new batch of widgets each month. We know the product codes of these widgets which will be is stock, waiting to be used. Each also has an availability date, after which it can be used.
Table WidgetStock
Columns: WidgetID, AvailabilityDate,
Another table has the uses of the widget, ie when it was first used.
Table WidgetsUsed
Columns: datetime, Operator
I'd like to see by day and hour the amount of fresh widgets that I have used for the first time, since the start of the month. The widgets will be used multiple times, so a simple distinct count by hour is not really enough, as widgets would be double counted.
In my mind this would require looking at a list of available numbers, which is updated for each line of a group by query.
The below query will not work, but hopefully it shows what I am trying to achieve:
declare #StartofMonth datetime
set #StartofMonth = '20160901'
select CONVERT(varchar, wu.datetime, 103)'Date'
, convert(char(2), wu.datetime, 108)'Hour'
--Problem Line below
, SUM(case when wu.StockNo in (select ba.NUMBER
from widgetStock ba
where availability_date between CONVERT(varchar, wu.datetime, 103) and #StartofMonth) then 1 else 0 end) 'Number Used'
from widgetsUsed wu
left join widgetStock ws on wu.StockNo = ws.NUMBER
where wu.OPERATOR = 'WidgetWorld'
and DATETIME between '20160914' and '20160916'
group by CONVERT(varchar,wu.datetime,103), convert(char(2), wu.datetime, 108)
Any help with this is appreciated. Thanks in advance.

Answer 1

If understood your requirement then below script may solve your problem
declare #StartofMonth datetime
set #StartofMonth = '20160901'
select CONVERT(varchar, wu.datetime, 103)'Date'
, convert(char(2), wu.datetime, 108)'Hour'
, SUM(ISNULL(ba.NUMBER,0)) 'Number Used'
from widgetsUsed wu
left join widgetStock ws on wu.StockNo = ws.NUMBER
LEFT JOIN widgetStock ba ON wu.StockNo = ba.NUMBER AND availability_date between CONVERT(varchar, wu.datetime, 103) and #StartofMonth
where wu.OPERATOR = 'WidgetWorld'
and DATETIME between '20160914' and '20160916'
group by CONVERT(varchar,wu.datetime,103), convert(char(2), wu.datetime, 108)