-- 学生表
CREATE TABLE students (
student_id INT PRIMARY KEY,
name VARCHAR(50),
age INT,
gender ENUM('男','女'),
enrollment_date DATE
);
INSERT INTO students VALUES
(1, '张三', 20, '男', '2022-09-01'),
(2, '李四', 22, '男', '2021-09-01'),
(3, '王五', 19, '女', '2022-09-01'),
(4, '赵六', 21, '女', '2021-09-01'),
(5, '钱七', 23, '男', '2020-09-01');
-- 课程表
CREATE TABLE courses (
course_id INT PRIMARY KEY,
course_name VARCHAR(50),
credit INT
);
INSERT INTO courses VALUES
(101, '数学', 4),
(102, '英语', 3),
(103, '物理', 4),
(104, '化学', 3);
-- 选课表(关联表)
CREATE TABLE enrollments (
enrollment_id INT PRIMARY KEY,
student_id INT,
course_id INT,
score DECIMAL(5,2),
enroll_date DATE,
FOREIGN KEY (student_id) REFERENCES students(student_id),
FOREIGN KEY (course_id) REFERENCES courses(course_id)
);
INSERT INTO enrollments VALUES
(1, 1, 101, 85.5, '2023-02-01'),
(2, 1, 102, 78.0, '2023-02-01'),
(3, 2, 101, 92.0, '2023-02-01'),
(4, 2, 103, 88.5, '2023-02-01'),
(5, 3, 102, 95.0, '2023-02-01'),
(6, 4, 104, 76.5, '2023-02-01'),
(7, 5, 101, 81.0, '2023-02-01'),
(8, 5, 104, 90.0, '2023-02-01');
-- 教师表
CREATE TABLE teachers (
teacher_id INT PRIMARY KEY,
name VARCHAR(50),
course_id INT,
FOREIGN KEY (course_id) REFERENCES courses(course_id)
);
INSERT INTO teachers VALUES
(1, '刘老师', 101),
(2, '杨老师', 102),
(3, '陈老师', 103),
(4, '黄老师', 104); 查询每门课程的平均成绩
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