mysql数据库查询的习题和答案

以下是一些MySQL数据库查询练习题及其答案：

1. 查询所有学生的姓名和年龄。

SELECT name, age FROM students;

2. 查询所有科目的名称和学分。

SELECT subject_name, credits FROM subjects;

3. 查询所有学生的姓名和所在班级的名称。

SELECT students.name, classes.class_name 
FROM students 
JOIN classes ON students.class_id = classes.class_id;

4. 查询所有选修了“数据库”课程的学生的姓名和所在班级的名称。

SELECT students.name, classes.class_name 
FROM students 
JOIN classes ON students.class_id = classes.class_id 
JOIN enrollments ON students.student_id = enrollments.student_id 
JOIN courses ON enrollments.course_id = courses.course_id 
WHERE courses.course_name = "数据库";

5. 查询所有学生的姓名和选修的所有课程的名称。

SELECT students.name, GROUP_CONCAT(courses.course_name SEPARATOR ',') AS courses_taken 
FROM students 
JOIN enrollments ON students.student_id = enrollments.student_id 
JOIN courses ON enrollments.course_id = courses.course_id 
GROUP BY students.student_id;

6. 查询所有学生的姓名和选修课程的总学分数。

SELECT students.name, SUM(subjects.credits) AS total_credits 
FROM students 
JOIN enrollments ON students.student_id = enrollments.student_id 
JOIN courses ON enrollments.course_id = courses.course_id 
JOIN subjects ON courses.subject_id = subjects.subject_id 
GROUP BY students.student_id;

7. 查询所有学生的姓名和平均成绩（假设每个学生的每个课程只有一次考试）。

SELECT students.name, AVG(grades.grade) AS average_grade 
FROM students 
JOIN enrollments ON students.student_id = enrollments.student_id 
JOIN courses ON enrollments.course_id = courses.course_id 
JOIN grades ON enrollments.enrollment_id = grades.enrollment_id 
GROUP BY students.student_id;

以上是一些较为基础的MySQL数据库查询练习题及其答案，希望对你有所帮助！