Untitled

For each row returned, also return a list of rows that match on some other columns
StudentID ClassID
a          1
b          1
c          1
a          2
a          3
c          2
b          3

StudentID  ClassID  Classmates
a          1        a,b,c
a          2        a,c
a          3        a,b

SELECT * FROM
(SELECT *, GROUP_CONCAT(StudentID)
FROM enrolled GROUP BY studentID, ClassID)
AS temporary WHERE temporary.StudentID=a

select e.studentid, e.classid, dt.classmates
from (
    select classid, group_concat(studentid) as classmates
    from enrolled
    group by classid
) as dt
join enrolled e on e.classid = dt.classid
where e.studentid = 'a'

+-----------+---------+------------+
| studentid | classid | classmates |
+-----------+---------+------------+
| a         |       1 | a,b,c      |
| a         |       2 | a,c        |
| a         |       3 | a,b        |
+-----------+---------+------------+

+-----------+---------+------------+
| studentid | classid | classmates |
+-----------+---------+------------+
| b         |       1 | a,b,c      |
| b         |       3 | a,b        |
+-----------+---------+------------+

+-----------+---------+------------+
| studentid | classid | classmates |
+-----------+---------+------------+
| c         |       1 | a,b,c      |
| c         |       2 | a,c        |
+-----------+---------+------------+