CREATE TABLE sc

(

  id    INT,

  name  VARCHAR(20),

  class VARCHAR(20),

  score INT

);



INSERT INTO sc VALUES (11, 'zx', '一年1班', 68);

INSERT INTO sc VALUES (12, 'zx1', '一年1班', 98);

INSERT INTO sc VALUES (13, 'zx2', '一年1班', 58);

INSERT INTO sc VALUES (14, 'zx3', '一年1班', 48);

INSERT INTO sc VALUES (15, 'zx4', '一年2班', 88);

INSERT INTO sc VALUES (16, 'zx5', '一年2班', 68);

INSERT INTO sc VALUES (17, 'zx6', '一年2班', 98);

INSERT INTO sc VALUES (18, 'zx7', '一年3班', 78);

INSERT INTO sc VALUES (19, 'zx8', '一年2班', 58);

INSERT INTO sc VALUES (20, 'zx9', '一年1班', 88);

INSERT INTO sc VALUES (21, 'zx10', '一年3班', 83);

INSERT INTO sc VALUES (22, 'zx11', '一年3班', 82);

INSERT INTO sc VALUES (23, 'zx12', '一年3班', 81);



SELECT

  s1.name,

  s1.class,

  s1.score

FROM sc s1

WHERE

  (

    SELECT count(1)

    FROM sc s2

    WHERE

      s1.class = s2.class AND s2.score >= s1.score

  ) <= 3

ORDER BY s1.class, s1.score DESC;
 

優化：使用儲存過程或函式來分批查詢