How to join to inner query and calculate column based on different groupings? - sql

Question

I have a table that contains data about a series of visits to shops.
The raw data for these visits can be found here.
My main table will have 1 row per Country, and will use something along the lines of:
Select Distinct o.Country from OtherTable as o
I need to add a new column to my main table, that uses the following calculation:
"Avg Visits by User" = (Sum of (No. Call IDs / No. unique User IDs)
for each day) / No. unique of days (based on Actual Start) for the
row.
I have formed this additional select statement to get the number of calls and users by day - but I am struggling to join this to my main table:
Select DATEPART(DAY, c.ActualStart) As 'Day',
CAST(CAST(COUNT(c.CallID) AS DECIMAL (5,1))/CAST(COUNT(Distinct c.UserID) AS DECIMAL (5,1)) AS DECIMAL (5,1)) as 'Value' from CallInfo as c
where (c.Status = 3))
Group by DATEPART(DAY, c.ActualStart)
For the country GB, I would expect to come to the see the following output:
Day Calls Users Calls / Users
13-Jun 29 8 3.625
14-Jun 31 7 4.428571429
So, in my main table, the calculation for my new column would be:
8.053571 / 2
Therefore, if I somehow add this to my table I would expect the following output:
Country Unique Days Sum of Calls/Users for each day) Final Calc
GB 2 8.053571429 4.026785714
I have tried adding this as a join, but I don't know how to join this to my main table. I could for example join on Call Id - but this would require the addition of a callID column in my inner query, and this would mean that the values are incorrect.

Answer 1

You can use a subquery to make calculations by day and after that make calculations by country. The result SQL query can be like this:
-- Make calculation by country, from the subquery
SELECT Country, UniqueDays = count(TheDay), CallsUserPerDay = sum(CallsPerUser),
FinalCalc = sum(CallsPerUser) / cast(count(TheDay) as DECIMAL)
FROM (
-- SUBQUERY: Make calculations by day
SELECT c.Country, c.ActualStart as TheDay,
Calls = COUNT(c.CallID),
Users = COUNT(Distinct c.UserID),
COUNT(c.CallID)
/CAST(COUNT(Distinct c.UserID) AS DECIMAL) as CallsPerUser
FROM CallInfo as c
WHERE (c.Status = 3)
GROUP BY c.Country, c.ActualStart
) data
GROUP BY Country
Note: I avoid use precission on DECIMAL casting to avoid rounding on final result.