Question

我必须找出输出作为班级名称，每个班级的学生人数和班级中的平均分数？我的两张桌子都是 -

 CREATE TABLE class(Fields_ID INT, Name VARCHAR(20));
        INSERT INTO class(Fields_ID,Name) VALUES(30,'FIRST');
        INSERT INTO class(Fields_ID,Name) VALUES(31,'SECOND');
        INSERT INTO class(Fields_ID,Name) VALUES(32,'THIRD');
        INSERT INTO class(Fields_ID,Name) VALUES(33,'FOURTH');
        INSERT INTO class(Fields_ID,Name) VALUES(34,'FIFTH');
SELECT * FROM class;

CREATE TABLE student(Fields_ID INT, Name VARCHAR(20));
        INSERT INTO student(Fields_ID,Name) VALUES(30,'JYOTI');
        INSERT INTO student(Fields_ID,Name) VALUES(31,'KIRTI');
        INSERT INTO student(Fields_ID,Name) VALUES(32,'YOGITA');
        INSERT INTO student(Fields_ID,Name) VALUES(33,'RASHMI');
        INSERT INTO student(Fields_ID,Name) VALUES(34,'NUPUR');
SELECT * FROM student;

CREATE TABLE Marks(Fields_ID INT, Student_ID INT NOT NULL,marks INT NOT NULL);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (30,40,100);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (31,41,88);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (32,42,72);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (33,43,33);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (34,44,15);
SELECT * FROM Marks;

我试图从以下代码返回所需的内容，但我无法形成平均分数的逻辑。知道怎么做。我是MySql的初学者，所以我无法找到问题。

SELECT class.Name , COUNT(student.name),Marks.marks
From class INNER JOIN student INNER JOIN marks
    ON class.Fields_ID=student.Fields_ID=Marks.Fields_ID
GROUP BY class.Name;

Answer 1

  SELECT class.Name , COUNT(student.name),avg(Marks.marks) as average_mark
  From class 
  INNER JOIN student on class.Fields_ID = student.Fields_ID
  INNER JOIN marks  on marks.Fields_ID = student.Fields_ID
  GROUP BY class.Name;

Answer 2

试试这个

  SELECT c.Name , COUNT(s.name),m.marks
  From class c
  INNER JOIN student s on c.Fields_ID = s.Fields_ID
  INNER JOIN Marks m on m.Fields_ID = s.Fields_ID
  GROUP BY c.Name

DEMO HERE

如果您想按商标排序，可以添加order by m.marks。

Answer 3

试试这个：

SELECT class.Name, COUNT(student.Name), avg(Marks.marks)
FROM class 
JOIN student 
ON class.Fields_ID = student.Fields_ID
JOIN Marks
ON student.Student_ID = Marks.Student_ID
GROUP BY class.Name;

我对您的表和示例数据进行了一些更改，因为我认为这更符合逻辑。我已将 Student_ID 字段添加到表学生中。因为否则你没有办法唯一地识别你表学生中的行。目前您正在使用Fields_ID，但我认为这不正确。你想知道学生的标记而不是字段。

示例数据

 CREATE TABLE class(Fields_ID INT, Name VARCHAR(20));
        INSERT INTO class(Fields_ID,Name) VALUES(30,'FIRST');
        INSERT INTO class(Fields_ID,Name) VALUES(31,'SECOND');
        INSERT INTO class(Fields_ID,Name) VALUES(32,'THIRD');
        INSERT INTO class(Fields_ID,Name) VALUES(33,'FOURTH');
        INSERT INTO class(Fields_ID,Name) VALUES(34,'FIFTH');

CREATE TABLE student(Student_ID INT NOT NULL, Fields_ID INT, Name VARCHAR(20));
        INSERT INTO student(Student_ID, Fields_ID,Name) VALUES(40, 30,'JYOTI');
        INSERT INTO student(Student_ID, Fields_ID,Name) VALUES(41, 31,'KIRTI');
        INSERT INTO student(Student_ID,Fields_ID,Name) VALUES(42, 32,'YOGITA');
        INSERT INTO student(Student_ID,Fields_ID,Name) VALUES(43, 33,'RASHMI');
        INSERT INTO student(Student_ID,Fields_ID,Name) VALUES(44, 34,'NUPUR');


CREATE TABLE Marks(Fields_ID INT, Student_ID INT NOT NULL,marks INT NOT NULL);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (30,40,100);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (31,41,88);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (32,42,72);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (33,43,33);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (34,44,15);

SQLFIDDLE Demo

编辑：

我认为目前您的表格Marks会有PRIMARY KEY名为Fields_ID。这不起作用，因为会有比每个字段更多的标记。因此，我建议将表格PRIMARY KEY中的marks更改为Marks_ID int auto_increment。如果您不想创建seprate键来标识行并使用compiste键（PK：Student_ID，Fields_ID），则可以。 BTW不要忘记更改学生的主键。

这个表结构包含样本数据，包括主键：

 CREATE TABLE class(Fields_ID INT PRIMARY KEY, Name VARCHAR(20));
        INSERT INTO class(Fields_ID,Name) VALUES(30,'FIRST');
        INSERT INTO class(Fields_ID,Name) VALUES(31,'SECOND');
        INSERT INTO class(Fields_ID,Name) VALUES(32,'THIRD');
        INSERT INTO class(Fields_ID,Name) VALUES(33,'FOURTH');
        INSERT INTO class(Fields_ID,Name) VALUES(34,'FIFTH');

CREATE TABLE student(Student_ID INT NOT NULL PRIMARY KEY, Fields_ID INT, Name VARCHAR(20));
        INSERT INTO student(Student_ID, Fields_ID,Name) VALUES(40, 30,'JYOTI');
        INSERT INTO student(Student_ID, Fields_ID,Name) VALUES(41, 31,'KIRTI');
        INSERT INTO student(Student_ID,Fields_ID,Name) VALUES(42, 32,'YOGITA');
        INSERT INTO student(Student_ID,Fields_ID,Name) VALUES(43, 33,'RASHMI');
        INSERT INTO student(Student_ID,Fields_ID,Name) VALUES(44, 34,'NUPUR');


CREATE TABLE Marks(Marks_ID INT auto_increment PRIMARY KEY, Fields_ID INT, Student_ID INT NOT NULL,marks INT NOT NULL) ;
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (30,40,100);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (31,41,88);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (32,42,72);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (33,43,33);
        INSERT INTO Marks(Fields_ID,Student_ID,Marks) VALUES (34,44,15);

SQLFiddle demo

如何从三个或更多表中获取数据？

3 个答案: