MySQL 8.0

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CREATE TABLE student_sibling (student_id INT, sibling_student_id INT);
INSERT INTO student_sibling VALUES(1,              4    ),
(15,             4    ),
(2,              1    ),
(6,              15);
SELECT * FROM student_sibling;


student_id
sibling_student_id


1
4

15
4

2
1

6
15


WITH RECURSIVE CTE AS (SELECT student_id
                       FROM student_sibling
                       WHERE sibling_student_id=4
                       UNION
                       SELECT S.student_id
                       FROM CTE C
                       JOIN student_sibling S ON S.sibling_student_id = C.student_id)
SELECT student_id FROM CTE
ORDER BY student_id;


student_id


1

2

6

15