【sql: 练习题19 ，20】查询每门课程被选修的学生数，查询出只选修两门课程的学生学号和姓名

题目 19：查询每门课程被选修的学生数

SELECT courseid,COUNT(studentid) FROM student_score GROUP BY courseid;

运行：

练习题20 ：查询出只选修两门课程的学生学号和姓名

SELECT student.*, r.a FROM student,
(SELECT studentid,COUNT(courseid) AS a FROM student_score GROUP BY student_score.studentid)r
WHERE r.a =2 AND student.id = r.studentid ;

运行：