写出学生表，课程表，选课表的增，删，查，改的sql语句

1. 学生表的增、删、查、改的SQL语句：

- 增加学生：`INSERT INTO students (id, name, age, gender) VALUES ('001', '小明', 18, '男');`
- 删除学生：`DELETE FROM students WHERE id = '001';`
- 查询学生：`SELECT * FROM students WHERE id = '001';`
- 修改学生信息：`UPDATE students SET name = '小红', age = 19 WHERE id = '001';`

2. 课程表的增、删、查、改的SQL语句：

- 增加课程：`INSERT INTO courses (id, name, teacher) VALUES ('001', '数学', '张老师');`
- 删除课程：`DELETE FROM courses WHERE id = '001';`
- 查询课程：`SELECT * FROM courses WHERE id = '001';`
- 修改课程信息：`UPDATE courses SET name = '英语', teacher = '李老师' WHERE id = '001';`

3. 选课表的增、删、查、改的SQL语句：

- 增加选课记录：`INSERT INTO course_selection (student_id, course_id) VALUES ('001', '001');`
- 删除选课记录：`DELETE FROM course_selection WHERE student_id = '001' AND course_id = '001';`
- 查询选课记录：`SELECT * FROM course_selection WHERE student_id = '001';`
- 修改选课记录：`UPDATE course_selection SET course_id = '002' WHERE student_id = '001' AND course_id = '001';`