MySQL 8.0

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CREATE TABLE students_classrooms(
student_id INT,
classroom_id INT  );

INSERT INTO students_classrooms VALUES
(1,2),
(2,2),
(3,2),
(66,3),
(77,3),
(99,3);

Records: 6  Duplicates: 0  Warnings: 0


SELECT classroom_id,
       SUBSTRING_INDEX(GROUP_CONCAT(student_id ORDER BY RAND()),',',1) AS student
FROM students_classrooms 
WHERE classroom_id IN (2,3)
GROUP BY classroom_id;


classroom_id
student


2
3

3
77


INSERT INTO students_classrooms VALUES
(4,2),
(5,2),
(6,2),
(88,3),
(55,3),
(44,3);

Records: 6  Duplicates: 0  Warnings: 0


SELECT classroom_id,
       SUBSTRING_INDEX(GROUP_CONCAT(student_id ORDER BY RAND()),',',1) AS student
FROM students_classrooms 
WHERE classroom_id IN (2,3)
GROUP BY classroom_id;


classroom_id
student


2
6

3
99