How to perform multiple table calculation with joins and group by - sql

Question

I have two tables client and grouping. They look like this:
Client
C_id
C_grouping_id
Month
Profit
Grouping
Grouping_id
Month
Profit
The client table contains monthly profit for every client and every client belongs to a specific grouping scheme specified by C_grouping_id.
The grouping table contains all the groups and their monthly profits.
I'm struggling with a query that essentially calculates the monthly residual for every subscriber:
Residual= (Subscriber Monthly Profit - Grouping monthly Profit)*(average subscriber monthly profits for all months / average profits for all months for the grouping subscriber belongs to)
I have come up with the following query so far but the results seem to be incorrect:
SELECT client.C_id, client.C_grouping_Id, client.Month,
((client.Profit - grouping.profit) * (avg(client.Profit)/avg(grouping.profit))) as "residual"
FROM client
INNER JOIN grouping
ON "C_grouping_id"="Grouping_id"
group by client.C_id, client.C_grouping_Id,client.Month, grouping.profit
I would appreciate it if someone can shed some light on what I'm doing wrong and how to correct it.
EDIT: Adding sample data and desired results
Client
C_id C_grouping_id Month Profit
001 aaa jul 10$
001 aaa aug 12$
001 aaa sep 8$
016 abc jan 25$
016 abc feb 21$
Grouping
Grouping_id Month Profit
aaa Jul 30$
aaa aug 50$
aaa Sep 15$
abc Jan 21$
abc Feb 27$
Query Result:
C_ID C_grouping_id Month Residual
001 aaa Jul (10-30)*(10/31.3)=-6.38
... and so on for every month for avery client.

Answer 1

This can be done in a pretty straight forward way.
The main difficulty is obviously that you try to deal with different levels of aggregation at once (average of the group and the client as well as the current record).
This is rather difficult/clumsy with simple SELECT FROM GROUP BY-SQL.
But with analytical functions aka Window functions this is very easy.
Start with combining the tables and calculating the base numbers:
select c.c_id as client_id,
c.c_grouping_id as grouping_id,
c.month,
c.profit as client_profit,
g.profit as group_profit,
avg (c.profit) over (partition by c.c_id) as avg_client_profit,
avg (g.profit) over (partition by g.grouping_id) as avg_group_profit
from client c inner join grouping g
on c."C_GROUPING_ID"=g."GROUPING_ID"
and c. "MONTH" = g. "MONTH";
With this you already get the average profits by client and by grouping_id.
Be aware that I changed the data type of the currency column to DECIMAL (10,3) as a VARCHAR with a $ sign in it is just hard to convert.
I also fixed the data for MONTHS as the test data contained different upper/lower case spellings which prevented the join to work.
Finally I turned all column names into upper case to, in order to make typing easier.
Anyhow, running this provides you with the following result set:
CLIENT_ID GROUPING_ID MONTH CLIENT_PROFIT GROUP_PROFIT AVG_CLIENT_PROFIT AVG_GROUP_PROFIT
16 abc JAN 25 21 23 24
16 abc FEB 21 27 23 24
1 aaa JUL 10 30 10 31.666
1 aaa AUG 12 50 10 31.666
1 aaa SEP 8 15 10 31.666
From here it's only one step further to the residual calculation.
You can either put this current SQL into a view to make it reusable for other queries or use it as a inline view.
I chose to use it as a common table expression (CTE) aka WITH clause because it's nice and easy to read:
with p as
(select c.c_id as client_id,
c.c_grouping_id as grouping_id,
c.month,
c.profit as client_profit,
g.profit as group_profit,
avg (c.profit) over (partition by c.c_id) as avg_client_profit,
avg (g.profit) over (partition by g.grouping_id) as avg_group_profit
from client c inner join grouping g
on c."C_GROUPING_ID"=g."GROUPING_ID"
and c. "MONTH" = g. "MONTH")
select client_id, grouping_id, month,
client_profit, group_profit,
avg_client_profit, avg_group_profit,
round( (client_profit - group_profit)
* (avg_client_profit/avg_group_profit), 2) as residual
from p
order by grouping_id, month, client_id;
Notice how easy to read the whole statement is and how straight forward the residual calculation is done.
The result is then this:
CLIENT_ID GROUPING_ID MONTH CLIENT_PROFIT GROUP_PROFIT AVG_CLIENT_PROFIT AVG_GROUP_PROFIT RESIDUAL
1 aaa AUG 12 50 10 31.666 -12
1 aaa JUL 10 30 10 31.666 -6.32
1 aaa SEP 8 15 10 31.666 -2.21
16 abc FEB 21 27 23 24 -5.75
16 abc JAN 25 21 23 24 3.83
Cheers,
Lars