【sql 练习题 38 ，39】检索至少选修两门课程的学生学号，查询选修了全部课程的学生信息

题目38：检索至少选修两门课程的学生学号

SELECT studentid ,COUNT(courseid) AS a
FROM student_score GROUP BY studentid HAVING a >=2

题目39：查询选修了全部课程的学生信息

GROUP BY studentid 然后count(score)=全部课程数

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