Find duplicates across many to many relationship in SQL Server - sql-server

Question

I am trying to find potential duplicates in a many-to-many join in a SQL Server database.
I have a database of students attending classes and have the following tables: Lessons, Attendees, Classrooms and Students.
I am trying to find duplicates where the same group of students may have been entered twice for the same date and classroom.
Students to Lessons is many-to-many broken down by the Attendees table. The LessonID, StudentID, ClassroomID fields are SQL Server Identity primary keys. Attendees is simply the join table with a compound key of student and lesson.
Lessons:
LessonID
LessonDate
ClassroomID
Students:
StudentID
Attendees:
LessonID
StudentID
Classrooms:
ClassroomID
It is legitimate that the same group of students may have attended different classes on the same day in the same classroom, but I want to flag them up as potential duplicates, in case the record has erroneously been entered twice.
I can’t figure out how to find matching sets of students for the same classroom on the same date.
So, an example of duplicate data I would expect to find would be:
Lessons:
+----------+-------------+------------+
| LessonID | ClassroomID | LessonDate |
+----------+-------------+------------+
| 335867 | 347 | 06/01/2020 |
| 335872 | 347 | 06/01/2020 |
+----------+-------------+------------+
Attendees:
+----------+----------+
| LessonID | PersonID |
+----------+----------+
| 335867 | 432 |
| 335867 | 1398 |
| 335867 | 5107 |
| 335872 | 432 |
| 335872 | 1398 |
| 335872 | 5107 |
+----------+----------+
Another way to look at this would be: for any given Lesson, which other lessons (if any) have the same students in the same classroom on the same day.

Answer 1

I found a solution myself using the STRING_AGG function to flatten out the hierarchy. I added the following query to the database:
SELECT Lessons.LessonID, Lessons.ClassroomID, Lessons.LessonDate, string_agg(Attendees.StudentID, '-') AS team
FROM Lessons INNER JOIN
Attendees ON Lessons.LessonID = Attendees.LessonID
GROUP BY Lessons.LessonID, Lessons.ClassroomID, Lessons.LessonDate
This gives lesson data that looks like this:
+---+----+------------+--------------+
| 1 | 17 | 2006-01-04 | 3-5-10-23 |
| 2 | 18 | 2006-01-04 | 2-17-252 |
| 3 | 18 | 2006-01-04 | 2-16-18 |
| 4 | 18 | 2006-01-04 | 2-6-11-16-18 |
+---+----+------------+--------------+
which I can then simply query against.
I will turn this into a stored procedure passing in for my chosen lesson: LessonDate, ClassroomID and its own "STRING_AGG" team of students, as filters.
The STRING_AGG function is only available from SQL Server 2017. So for older versions you can use the FOR XML PATH('') syntax, concatenating with a hyphen, with a STUFF to remove the leading hyphen:
SELECT dbo.Lessons.LessonID, dbo.Lessons.ClassroomID, dbo.Lessons.LessonDate,
(
stuff(
(select '-' + cast(StudentId as varchar(10))
FROM Attendees
WHERE Attendees.LessonId = Lessons.Lessonid
FOR XML path('')
),1,1,'')
)
as Team
FROM dbo.Lessons
You could concatenate with a comma instead for standard CSV format if preferred.