数据库MySQL查询学生的学号、姓名、选修课程的课程名及成绩。（3表连接）

假设我们有三张表：`students`、`courses`和`scores`，分别存储学生信息、课程信息和成绩信息。它们的结构如下：

students表：
+----+--------+
| id | name   |
+----+--------+
| 1  | Alice  |
| 2  | Bob    |
| 3  | Charlie|
+----+--------+

courses表：
+----+-----------+
| id | name      |
+----+-----------+
| 1  | Math      |
| 2  | Physics   |
| 3  | Chemistry |
+----+-----------+

scores表：
+----+------------+--------+-------+
| id | student_id | course_id | score |
+----+------------+----------+-------+
| 1  | 1          | 1        | 90    |
| 2  | 1          | 2        | 85    |
| 3  | 2          | 1        | 95    |
| 4  | 2          | 3        | 80    |
| 5  | 3          | 2        | 92    |
| 6  | 3          | 3        | 88    |
+----+------------+----------+-------+

我们可以通过以下 SQL 语句查询学生的学号、姓名、选修课程的课程名及成绩：

SELECT students.id, students.name, courses.name, scores.score
FROM students
INNER JOIN scores ON students.id = scores.student_id
INNER JOIN courses ON scores.course_id = courses.id;

运行结果如下：

+----+--------+-----------+-------+
| id | name   | name      | score |
+----+--------+-----------+-------+
| 1  | Alice  | Math      | 90    |
| 1  | Alice  | Physics   | 85    |
| 2  | Bob    | Math      | 95    |
| 2  | Bob    | Chemistry | 80    |
| 3  | Charlie| Physics   | 92    |
| 3  | Charlie| Chemistry | 88    |
+----+--------+-----------+-------+