LeetCode_db:查出每個班級分數前3高的學生

需求：
查出每個班級分數前3高的學生

CREATE TABLE `stu` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) DEFAULT '' COMMENT '學生名',
  `class` int(11) DEFAULT NULL COMMENT '班級',
  `score` int(11) DEFAULT NULL COMMENT '分數',
  PRIMARY KEY (`ID`),
  UNIQUE KEY `unique_idx_name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COMMENT='學生';

INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('1', 'a', '1', '88');
INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('2', 'b', '1', '99');
INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('3', 'c', '2', '99');
INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('4', 'd', '2', '97');
INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('5', 'e', '2', '97');
INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('6', 'f', '2', '98');
INSERT INTO `mytest`.`stu` (`ID`, `name`, `class`, `score`) VALUES ('7', 'g', '2', '55');

SELECT
   *
FROM
   stu
WHERE
   `name` IN (
      SELECT
         a.`name`
      FROM
         stu a
      LEFT JOIN stu b ON a.class = b.class
      AND a.score < b.score
      GROUP BY
         a.`name`
      HAVING
         COUNT(DISTINCT b.`name`) <= 2
   )
ORDER BY
   class,
   score DESC

輸出：
ID name   class  score
2  b      1      99
1  a      1      88
3  c      2      99
6  f      2      98
4  d      2      97
5  e      2      97