SSRS - Repeated values in Matrix - reporting-services

Question

I am very much a beginner at this, so please pardon my ignorance. I have searched for hours, but don't yet have the experience to have a better idea of precisely what to search for!
I have a data set in SSRS as follows:
|---------------------|------------------|
| Subject | Student |
|---------------------|------------------|
| Biology | Student A |
|---------------------|------------------|
| Chemistry | Student A |
|---------------------|------------------|
| Chemistry | Student B |
|---------------------|------------------|
| Physics | Student B |
|---------------------|------------------|
I want to write a matrix which counts all the students who are doing a combination of subjects (eg. Chemistry and Physics). Currently, my matrix is only counting the number of students in an individual subject, leading to something like this:
|---------------------|------------------|------------------|------------------|
| | Biology | Chemistry | Physics |
|---------------------|------------------|------------------|------------------|
| Biology | 20 | 0 | 0 |
|---------------------|------------------|------------------|------------------|
| Chemistry | 0 | 36 | 0 |
|---------------------|------------------|------------------|------------------|
| Physics | 0 | 0 | 16 |
|---------------------|------------------|------------------|------------------|
What am I missing?

Answer 1

First, your dataset needs to combine subjects as Subject1 and Subject2, for example by joining the table to itself on the Student field.
Then, in the matrix, use a row group that groups on Subject1 and a column group that groups on Subject2.

Answer 2

That's easy:
You inner join the map between student and subject with itselfs, but only if the student is the same, but the subject is different.
Then you do CountDistinct on the studentId in the report, doesn't matter if it's from map1 or map2, since it's always the same.
;WITH subjects AS
(
SELECT 1 AS subject_id, 'Biology' AS subject_name
UNION ALL SELECT 2 AS subject_id, 'Chemistry' AS subject_name
UNION ALL SELECT 3 AS subject_id, 'Physics' AS subject_name
)
,students AS
(
SELECT 1 AS student_id, 'Student A' AS student_name
UNION ALL SELECT 2 AS student_id, 'Student B' AS student_name
UNION ALL SELECT 3 AS student_id, 'Student C' AS student_name
UNION ALL SELECT 4 AS student_id, 'Student D' AS student_name
)
,map_student_subject AS
(
SELECT 1 AS student_id, 1 AS subject_id
UNION ALL SELECT 1 AS student_id, 2 AS subject_id
UNION ALL SELECT 2 AS student_id, 2 AS subject_id
UNION ALL SELECT 2 AS student_id, 3 AS subject_id
UNION ALL SELECT 3 AS student_id, 3 AS subject_id
UNION ALL SELECT 4 AS student_id, 3 AS subject_id
)
SELECT
map1.student_id AS map1_student_id
,map1_student.student_name AS map1_student_name
,map1.subject_id AS map1_subject_id
,map1_subject.subject_name AS map1_subject_name
,map2.student_id AS map2_student_id
,map2_student.student_name AS map2_student_name
,map2.subject_id AS map2_subject_id
,map2_subject.subject_name AS map2_subject_name
FROM map_student_subject AS map1
INNER JOIN map_student_subject AS map2
ON map2.student_id = map1.student_id
AND map2.subject_id <> map1.subject_id
LEFT JOIN students AS map1_student
ON map1_student.student_id = map1.student_id
LEFT JOIN students AS map2_student
ON map2_student.student_id = map2.student_id
LEFT JOIN subjects AS map1_subject
ON map1_subject.subject_id = map1.subject_id
LEFT JOIN subjects AS map2_subject
ON map2_subject.subject_id = map2.subject_id
Result:
And if you want to list the students that have just one subject as well, you'll need to make a UNION, because just removing the <> will make it impossible to calculate the actual number of students in just one subject:
;WITH subjects AS
(
SELECT 1 AS subject_id, 'Biology' AS subject_name
UNION ALL SELECT 2 AS subject_id, 'Chemistry' AS subject_name
UNION ALL SELECT 3 AS subject_id, 'Physics' AS subject_name
)
,students AS
(
SELECT 1 AS student_id, 'Student A' AS student_name
UNION ALL SELECT 2 AS student_id, 'Student B' AS student_name
UNION ALL SELECT 3 AS student_id, 'Student C' AS student_name
UNION ALL SELECT 4 AS student_id, 'Student D' AS student_name
)
,map_student_subject AS
(
SELECT 1 AS student_id, 1 AS subject_id
UNION ALL SELECT 1 AS student_id, 2 AS subject_id
UNION ALL SELECT 2 AS student_id, 2 AS subject_id
UNION ALL SELECT 2 AS student_id, 3 AS subject_id
UNION ALL SELECT 3 AS student_id, 3 AS subject_id
UNION ALL SELECT 4 AS student_id, 3 AS subject_id
)
SELECT
map1.student_id AS map1_student_id
,map1_student.student_name AS map1_student_name
,map1.subject_id AS map1_subject_id
,map1_subject.subject_name AS map1_subject_name
,map2.student_id AS map2_student_id
,map2_student.student_name AS map2_student_name
,map2.subject_id AS map2_subject_id
,map2_subject.subject_name AS map2_subject_name
FROM map_student_subject AS map1
INNER JOIN map_student_subject AS map2
ON map2.student_id = map1.student_id
AND map2.subject_id <> map1.subject_id
LEFT JOIN students AS map1_student
ON map1_student.student_id = map1.student_id
LEFT JOIN students AS map2_student
ON map2_student.student_id = map2.student_id
LEFT JOIN subjects AS map1_subject
ON map1_subject.subject_id = map1.subject_id
LEFT JOIN subjects AS map2_subject
ON map2_subject.subject_id = map2.subject_id
UNION ALL
SELECT
students.student_id AS map1_student_id
,students.student_name AS map1_student_name
,MAX(subjects.subject_id) AS map1_subject_id
,MAX(subjects.subject_name) AS map1_subject_name
,students.student_id AS map2_student_id
,students.student_name AS map2_student_name
,MAX(subjects.subject_id) AS map2_subject_id
,MAX(subjects.subject_name) AS map2_subject_name
FROM map_student_subject
LEFT JOIN students ON students.student_id = map_student_subject.student_id
LEFT JOIN subjects ON subjects.subject_id = map_student_subject.subject_id
GROUP BY
students.student_id
,students.student_name
HAVING COUNT(*) = 1