插入多条数据：

INSERT INTO scores (`StuId`, `Course`, `Scores`) 
VALUES
  (9, '数学', 78),
  (9, '语文', 635),
  (9, '英语', 69) ;

INSERT INTO classinfo (`ClassId`, `ClassName`) 
VALUES
  (1, '数学'),
  (2, '语文'),
  (3, '英语') ;
  
  
INSERT INTO stuinfo (`StuId`, `ClassId`, `StuName`) 
VALUES
  (7, 3,'七'),
  (8, 3, '八'),
  (9, 3, '九') ;
  
  
  
  

  #目标：查找每个班的最高分：
  
SELECT 
  sm.id,
  sm.StuName,
  sm.ClassId,
  sm.ClassName,
  sc.`Course`,
  MAX(sc.`Scores`) 
FROM
  (SELECT 
    st1.`StuId` AS id,
    c1.`ClassId`,
    c1.`ClassName`,
    st1.`StuName` 
  FROM
    classinfo c1,
    stuinfo st1 
  WHERE c1.`ClassId` = st1.`ClassId` 
  ORDER BY st1.`StuId`) AS sm 
  LEFT JOIN scores sc 
    ON sc.`StuId` = sm.`id` 
GROUP BY sm.ClassName ;

  #目标：查找每个班的最高分：
  
SELECT 
  sm.id,
  sm.StuName,
  sm.ClassId,
  sm.ClassName,
  sc.`Course`,
  MAX(sc.`Scores`) 
FROM
  (SELECT 
    st1.`StuId` AS id,
    c1.`ClassId`,
    c1.`ClassName`,
    st1.`StuName` 
  FROM
    classinfo c1 LEFT JOIN
    stuinfo st1 
  ON c1.`ClassId` = st1.`ClassId` ) AS sm 
  LEFT JOIN scores sc 
    ON sc.`StuId` = sm.`ID` 
GROUP BY sm.ClassId;

SELECT c.StuId,c.ClassId,c.StuName,c.ClassName,d.Course,MAX(d.Scores) -- 3)显示学生信息

FROM

#111111111111111111111
(SELECT a.StuId,a.ClassId,a.StuName,b.ClassName FROM stuinfo a

LEFT JOIN classinfo b -- 1)首先关联学生表a 和班级表b

ON a.ClassId = b.ClassId) c

LEFT JOIN scores d -- 2)学生和班级的关联表c 再与成绩表d关联

ON c.StuId = d.StuId

GROUP BY c.StuId -- 4)根据班级分组


#22222222222222

SELECT A.StuId,A.ClassId,A.StuName,B.ClassName,MAX(C.Scores),C.Course FROM stuinfo A

LEFT JOIN classinfo B ON A.ClassID=B.ClassId

LEFT JOIN scores C ON A.StuId=C.StuId

GROUP BY B.ClassId