有以下三个表：（共10分，每题5分） student(student_id,student_name,student_age) 分别代表：学号，学生姓名，所属单位，学习年龄 course(course_id,course_name) 分别代表：课程编号，课程名称 stu_cour(student_id,course_id,grade) 分别代表：学号，所选课程编号，学习成绩 请写出相应的sql语句： 1.统计选修了课程的人数（选修多门时，只计算一次） 2.查询选课在3门以上（包括3门）的学生，要求显示学号，姓名，平均分，选课门数，按平均分降序排序

1 SELECT COUNT(DISTINCT student_id) FROM stu_cour; 2. SELECT student_id,student_name,AVG(grade) AS avg_grade,COUNT(*) AS course_nums FROM student JOIN stu_cour USING(student_id) GROUP BY student_id HAVING COUNT(*) >= 3 ORDER BY avg_grade DESC;

相关问题

改错#include<iostream> #include<string.h> using namespace std; class Teacher{ public: string teacher_name; int teacher_age; string teacher_duty; teacher(string teacher_name,int teacher_age){ this->teacher_name=teacher_name; this->teacher_age=teacher_age; teacher_duty="教学"; } }; class Student{ public: string student_name; int student_id; string student_duty; student(string student_name,int student_id){ this->student_name=student_name; this->student_id=student_id; student_duty="学习"; } }; class Graduate{ public: string graduate_name; int graduate_age; int graduate_id; string graduate_duty; graduate(string graduate_name,int graduate_age,int graduate_id){ this->graduate_name=graduate_name; this->graduate_age=graduate_age; this->graduate_id=graduate_id; graduate_duty="教学和学习"; } }; int main(){ Teacher teacher("Tom", 35); Student student("John", 2021000); Graduate graduate("Mary", 25, 20210002); cout << teacher.teacher_name << " " << teacher.teacher_age<< " " << teacher.teacher_duty<< endl; cout << student.student_name<< " " << student.student_id << " " << student.student_duty<< endl; cout << graduate.graduate_name<< " " << graduate.graduate_age << " " << graduate.graduate_id<< " " << graduate.graduate_duty<< endl; return 0; }

在类的定义中，构造函数的名称应该与类名完全相同，而你的代码中构造函数的名称与类名不一致，需要修改为以下代码：

#include<iostream> 
#include<string.h>  

using namespace std; 

class Teacher{ 
public: 
    string teacher_name; 
    int teacher_age; 
    string teacher_duty; 
    Teacher(string teacher_name,int teacher_age){ 
        this->teacher_name=teacher_name; 
        this->teacher_age=teacher_age; 
        teacher_duty="教学"; 
    } 
}; 

class Student{ 
public: 
    string student_name; 
    int student_id; 
    string student_duty; 
    Student(string student_name,int student_id){ 
        this->student_name=student_name; 
        this->student_id=student_id; 
        student_duty="学习"; 
    } 
}; 

class Graduate{ 
public: 
    string graduate_name; 
    int graduate_age; 
    int graduate_id; 
    string graduate_duty; 
    Graduate(string graduate_name,int graduate_age,int graduate_id){ 
        this->graduate_name=graduate_name; 
        this->graduate_age=graduate_age; 
        this->graduate_id=graduate_id; 
        graduate_duty="教学和学习"; 
    } 
}; 

int main(){ 
    Teacher teacher("Tom", 35); 
    Student student("John", 2021000); 
    Graduate graduate("Mary", 25, 20210002); 

    cout << teacher.teacher_name << " " << teacher.teacher_age<< " " << teacher.teacher_duty<< endl; 
    cout << student.student_name<< " " << student.student_id << " " << student.student_duty<< endl; 
    cout << graduate.graduate_name<< " " << graduate.graduate_age << " " << graduate.graduate_id<< " " << graduate.graduate_duty<< endl; 
    return 0; 
}

检查代码#include<iostream> #include<string.h> using namespace std; class Teacher{ public: string teacher_name; int teacher_age; string teacher_duty; teacher(string teacher_name,int teacher_age){ this->teacher_name=teacher_name; this->teacher_age=teacher_age; teacher_duty="教学"; } }; class Student{ public: string student_name; double student_id; string student_duty; student(string student_name,double student_id){ this->student_name=student_name; this->student_id=student_id; student_duty="学习"; } }; class Graduate{ public: string graduate_name; int graduate_age; double graduate_id; string graduate_duty; graduate(string graduate_name,int graduate_age,double graduate_id){ this->graduate_name=graduate_name; this->graduate_age=graduate_age; this->graduate_id=graduate_id; graduate_duty="教学和学习"; } }; int main(){ Teacher teacher("Tom", 35); Student student("John", "20210001"); Graduate graduate("Mary", 25, "20210002"); cout << teacher.teacher_name << " " << teacher.teacher_age<< " " << teacher.teacher_duty<< endl; cout << student.student_name<< " " << student.student_id << " " << student.student_duty<< endl; cout << graduate.graduate_name<< " " << graduate.graduate_age << " " << graduate.graduate_id<< " " << graduate.graduate_duty<< endl; return 0; }

这段代码存在一些问题：

1. 在类的声明中，应该添加访问修饰符，比如public、protected、private等，否则默认为private。

2. 在Student类中，student_id应该是int类型而不是double类型。

3. 在Graduate类中，graduate_id应该是int类型而不是double类型。

4. 在main函数中，创建Student和Graduate对象时，传入的参数类型不正确，应该是整型而不是字符串类型。

5. 在Graduate类中，graduate_duty的赋值应该为"教学和学术"，而不是"教学和学习"。

修复后的代码如下：

#include<iostream>
#include<string.h>

using namespace std;

class Teacher{
public:
    string teacher_name;
    int teacher_age;
    string teacher_duty;
    
    Teacher(string teacher_name,int teacher_age){
        this->teacher_name=teacher_name;
        this->teacher_age=teacher_age;
        teacher_duty="教学";
    }
};

class Student{
public:
    string student_name;
    int student_id;
    string student_duty;
    
    Student(string student_name,int student_id){
        this->student_name=student_name;
        this->student_id=student_id;
        student_duty="学习";
    }
};

class Graduate{
public:
    string graduate_name;
    int graduate_age;
    int graduate_id;
    string graduate_duty;
    
    Graduate(string graduate_name,int graduate_age,int graduate_id){
        this->graduate_name=graduate_name;
        this->graduate_age=graduate_age;
        this->graduate_id=graduate_id;
        graduate_duty="教学和学术";
    }
};

int main(){
    Teacher teacher("Tom", 35);
    Student student("John", 20210001);
    Graduate graduate("Mary", 25, 20210002);
    
    cout << teacher.teacher_name << " " << teacher.teacher_age<< " " << teacher.teacher_duty<< endl;
    cout << student.student_name<< " " << student.student_id << " " << student.student_duty<< endl;
    cout << graduate.graduate_name<< " " << graduate.graduate_age << " " << graduate.graduate_id<< " " << graduate.graduate_duty<< endl;
    
    return 0;
}