MySQL5 - 分組查詢中取各組的前N名

成績表格Grade

StudentId	Class	Score
1	A	30
2	A	60
3	A	75
4	A	31
5	A	25
6	B	95
7	B	33
8	B	57
9	B	87
10	C	22
11	C	17
12	C	85
12	C	90
14	C	66

查詢每個班級成績最高前兩名

# 建立table
CREATE TABLE `Grade` (
     `StudentId` int(11) NOT NULL AUTO_INCREMENT,
     `Class` varchar(100) DEFAULT NULL,
     `Score` int(11) DEFAULT NULL,
      PRIMARY KEY (`StudentId`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

#插入測試數據
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(1, 'A', 30);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(2, 'A', 60);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(3, 'A', 75);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(4, 'A', 31);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(5, 'A', 25);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(6, 'B', 95);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(7, 'B', 33);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(8, 'B', 57);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(9, 'B', 87);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(10, 'C', 22);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(11, 'C', 17);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(12, 'C', 85);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(13, 'C', 90);
INSERT INTO bdg_go_base.Grade (StudentId, Class, Score) VALUES(14, 'C', 66);

#查詢語句
SELECT 
     Grade.*
FROM 
     Grade INNER JOIN 
     (     SELECT 
                 Class, 
                  GROUP_CONCAT(StudentId ORDER BY Score DESC) gStudentId
            FROM 
                  Grade
            GROUP BY Class 
      )  gGrade
      ON 
            Grade.Class = gGrade.Class 
            AND FIND_IN_SET(Grade.StudentId, gGrade.gStudentId) BETWEEN 1 AND 2
ORDER BY
     Grade.Class, Grade.StudentId;

結果

StudentId	Class	Score
2	A	60
3	A	75
6	B	95
9	B	87
12	C	85
13	C	90

參考資料
https://stackoverflow.com/a/15585351