MySQL Subqueries

SELECT name, score FROM students
WHERE score > (SELECT AVG(score) FROM students);

SELECT s.name FROM students s
WHERE EXISTS (
    SELECT 1 FROM enrollments e WHERE e.student_id = s.id
);

SELECT s.name, e.score,
       e.score - (SELECT AVG(e2.score)
                  FROM enrollments e2
                  WHERE e2.course_id = e.course_id) AS diff_from_avg
FROM students s
JOIN enrollments e ON s.id = e.student_id;

SELECT * FROM (
    SELECT course_id, student_id, score,
           ROW_NUMBER() OVER (PARTITION BY course_id ORDER BY score DESC) AS rn
    FROM enrollments
) ranked WHERE rn <= 3;