Returning unique values in a MS Access query based upon a set a rules - sql

Question

I have a query that returns a dataset with UserID, CourseID, Course Completion date, Course Registration Date and Status (successful, not attempted, not finished).
However people are allowed to do a course multiple times.
For 1 report I need to get a unique recordset (based upon the Combination UserID and CourseID) back according to the following rules:
If a course is completed successfully by a learner take only that value
If a course is completed successfully by a learner multiple times, take the first completion date.
if a course not completed successfully by a learner take the last registration date.
I know how to create a query that only returns unique (Distinct) values, but not how to do it with a set of rules.

Answer 1

I see that this is a bit older. I don't know if this directly answers your question because I couldn't think of a way to write a single query to do this. I did however come up with a solution that I tested myself.
Using 3 tables and 4 queries, I think that I have produced the results you're looking for.
The first query I wrote was to get the "First Completed" dates. Since if a person only completes a course once, that is their first completion, this actually means your criteria 1 and 2 are identical.
I did this by grouping on the UserID and the CourseID and then taking the aggregate MIN of the CourseCompleteionDate where the Status is equal to 1.
SELECT tblRegistrations.UserID, tblRegistrations.CourseID, Min(tblRegistrations.CourseCompletionDate) AS FirstCompleted
FROM tblRegistrations
WHERE ((tblRegistrations.StatusID)=1)
GROUP BY tblRegistrations.UserID, tblRegistrations.CourseID;
The second query I wrote was to get the "Last Not Completed" registration dates. Again I grouped on the UserID and the CourseID, but this time I took the aggregate MAX of the CourseRegistrationDate to get the last time this course was attempted and not completed (with a status equal to 3)
SELECT tblRegistrations.UserID, tblRegistrations.CourseID, Max(tblRegistrations.CourseRegistrationDate) AS LastAttempt
FROM tblRegistrations
WHERE (((tblRegistrations.StatusID)=3))
GROUP BY tblRegistrations.UserID, tblRegistrations.CourseID;
The third query I wrote was to get the unique UserID/CourseID relationships, and that simply grouped them.
SELECT tblRegistrations.CourseID, tblRegistrations.UserID
FROM tblRegistrations
GROUP BY tblRegistrations.CourseID, tblRegistrations.UserID;
Finally, I wrote my result query which makes sure to pull every UserID/CourseID combination, and then show whether it was completed or not, and the requested date.
SELECT qryUserCourses.UserID, qryUserCourses.CourseID, Nz([FirstCompleted],[LastAttempt]) AS ResultDate, IIf([FirstCompleted],"Completed","Not Completed") AS Result
FROM (qryUserCourses
LEFT JOIN qryFirstCompleted ON (qryUserCourses.CourseID = qryFirstCompleted.CourseID) AND (qryUserCourses.UserID = qryFirstCompleted.UserID))
LEFT JOIN qryLastNotCompleted ON (qryUserCourses.CourseID = qryLastNotCompleted.CourseID) AND (qryUserCourses.UserID = qryLastNotCompleted.UserID);
This produced, for me, a list of UserIDs, CourseIDs, a date, and completion status.