SELECT s.*, 
       e1.id AS exp1_id, 
       e2.id AS exp2_id, 
       e3.id AS exp3_id
FROM students s
LEFT JOIN exp1 e1 ON s.id = e1.student
LEFT JOIN exp2 e2 ON s.id = e2.student
LEFT JOIN exp3 e3 ON s.id = e3.student
WHERE s.id IN (1, 2);