SQL for Microsoft Access 2013 - sql

Question

I am trying to get a count of customers who return for additional services that takes into account a comparison of assessment scores prior to the service, grouped by the type of Service. (Ultimately, I also want to be able to ignore returns that are within a month of the orginal service, but I'm pretty sure I can get that wrinkle sorted myself)
When counting results for a particular service, it should look at returns to any service type, not just the original service type. (Edit: *It should also look at all future returns, not just the next or the most recent *).
It does not need to be run often, but there are 15000+ lines of data and computational resources are limited by an underpowered machine (this is for a nonprofit organization), so efficiency would be nice but not absolutely needed.
Sample Data
ServiceTable
CustomerID Service Date ScoreBefore
A Service1 1/1/2017 1
A Service2 1/3/2017 1
A Service1 1/1/2018 4
B Service3 3/1/2018 3
B Service1 6/1/2018 1
B Service1 6/2/2018 1
C Service2 1/1/2019 4
C Service2 6/1/2019 1
Results should be (not taking into account the date padding option):
Service1
ReturnedWorse 0
ReturnedSame 2
ReturnedBetter 1
Service2
ReturnedWorse 1
ReturnedSame 0
ReturnedBetter 1
Service3
ReturnedWorse 2
So far, I have tried creating make table queries that could then be queried to get the aggregate info, but I am a bit stuck and suspect there may be a better route.
What I have tried:
SELECT CustomerID, Service, Date, ScoreBefore INTO ReturnedWorse
FROM ServiceTable AS FirstStay
WHERE ((((SELECT COUNT(*)
FROM ServiceTable AS SecondStay
WHERE FirstStay.CustomerID=SecondStay.CustomerID
AND
FirstStay.ScoreBefore> SecondStay.ScoreBefore
AND
SecondStay.Date > FirstStay.Date))));
Any help would be greatly appreciated.

Answer 1

This would have been easier to do with window functions, but they are not available in ms-access.
Here is a query that solves my understanding of your question :
t0: pick a record in the table (a customer buying a service)
t1 : pull out the record corresponding to the next time the same customer contracted any service with an INNER JOIN and a correlated subquery (if there is no such record, the initial record is not taken into account)
compare the score of the previous record to the current one
group the results by service id
You can see it in action in this db fiddlde. The results are slightly different from your expectation (see my comments)... but they are consistent with the above explanation ; you might want to adapt some of the rules to match your exact expected result, using the same principles.
SELECT
t0.service,
SUM(CASE WHEN t1.scorebefore < t0.scorebefore THEN 1 ELSE 0 END) AS ReturnedWorse,
SUM(CASE WHEN t1.scorebefore = t0.scorebefore THEN 1 ELSE 0 END) AS ReturnedSame,
SUM(CASE WHEN t1.scorebefore > t0.scorebefore THEN 1 ELSE 0 END) AS ReturnedBetter
FROM
mytable t0
INNER JOIN mytable t1
ON t0.customerid = t1.customerid
AND t0.date < t1.date
AND NOT EXISTS (
SELECT 1
from mytable
WHERE
customerid = t1.customerid
AND date < t1.date
AND date > t0.date
)
GROUP BY t0.service
| service | ReturnedWorse | ReturnedSame | ReturnedBetter |
| -------- | ------------- | ------------ | -------------- |
| Service1 | 0 | 2 | 0 |
| Service2 | 1 | 0 | 1 |
| Service3 | 1 | 0 | 0 |
From your comments, I understand that you want to take into account all future returns and not only the next one. This eliminates the need for a correlatead subquery, and actually yields your expected output. See this db fiddle :
SELECT
t0.service,
SUM(CASE WHEN t1.scorebefore < t0.scorebefore THEN 1 ELSE 0 END) AS ReturnedWorse,
SUM(CASE WHEN t1.scorebefore = t0.scorebefore THEN 1 ELSE 0 END) AS ReturnedSame,
SUM(CASE WHEN t1.scorebefore > t0.scorebefore THEN 1 ELSE 0 END) AS ReturnedBetter
FROM
mytable t0
INNER JOIN mytable t1
ON t0.customerid = t1.customerid
-- AND t0.service = t1.service
AND t0.date < t1.date
GROUP BY t0.service
| service | ReturnedWorse | ReturnedSame | ReturnedBetter |
| -------- | ------------- | ------------ | -------------- |
| Service1 | 0 | 2 | 1 |
| Service2 | 1 | 0 | 1 |
| Service3 | 2 | 0 | 0 |