mysql数据库有选课表learn(student_id int, course_id int)，字段分别表示学号和课程编号，现在想获取每个学生所选课程的个数信息，请问如下的sql语句正确的是
select student_id, sum(course_id) from learn
select student_id, count(course_id) from learn
select student_id, count(course_id) from learn group by student_id
select student_id, sum(course_id) from learn group by student_id
