Question

我必须找出每个班级的班级名称和学生人数？我的两张桌子都是 -

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 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;

我试图从以下代码返回所需的内容，但它不会返回相同的内容。知道为什么它没有返回正确的值。我是MySql的初学者，所以我无法找出问题。

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

Answer 1

使用聚合函数，你应该像GROUP BY class.Name一样对它们进行分组，这样你就可以得到每个班级的学生数量，另外你会得到一行而不是每组的结果，即每班学生

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

GROUP BY (Aggregate) Functions

See fiddle demo

如何从mysql中的两个表中获取数据？

1 个答案: