MySQL数据库多表查询练习题

原题目数据库请查看作者资源免费下载
(https://download.csdn.net/download/MeiJin_/86403712)

多表查询的思路 类似于生活日常中解决问题方式 第一步干什么第二步干什么
连表操作就是 先将所有涉及到结果的表全部拼接到一起形成一张大表 然后从大表中查询数据

1、查询所有的课程的名称以及对应的任课老师姓名

(看题 首先看题目我们会需要两张表 一个课程 一个教师 只需要拼接在一起获取到信息即可)

select course.cname,teacher.tname from course inner join teacher on course.teacher_id = teacher.tid

2、查询平均成绩大于八十分的同学的姓名和平均成绩

select student_id,avg(num) from score GROUP BY student_id;	# 拿到分数表获取每个学生的平均分

select student_id,avg(num) from score GROUP BY student_id HAVING AVG(num) > 80; # 获取平均分大于80的同学ID

select * from student INNER JOIN (select student_id,avg(num)as avg_num from score  # 注意如果不命名则无法取值
GROUP BY student_id HAVING AVG(num) > 80)as t1 ON student.sid = student_id; # 拿学生的分数id去跟学生表对接  

select student.sname, t1.avg_num from student inner join(select student_id,avg(num))as avg_num
FROM score GROUP BY student_id HAVING AVG( num ) > 80 ) AS t1 ON student.sid = student_id;		# 我们只需要学生的姓名以及成绩即可 

3、查询没有报李平老师课的学生姓名

select tid from teacher where tname ='李平老师'; # 先获取到老师的编号 2 

select cid from course WHERE teacher_id = (select tid from teacher where tname ='李平老师')	 # 再通过老师的编号去获取课程编号 2 4 

select DISTINCT student_id FROM score WHERE course_id in	# 根据课程编号 去分数表中筛选出选择了课程的学生编号 DISTINCT 去重复 	
(select cid from course WHERE teacher_id = (select tid from teacher where tname ='李平老师'))

select sname from student WHERE sid not IN
(select DISTINCT student_id FROM score WHERE course_id in 	# 通过学生的编号 在分数表中取反 看没有选择李平老师的课程学生姓名！
(select cid from course WHERE teacher_id = (select tid from teacher where tname ='李平老师')))

4、查询没有同时选修物理课程和体育课程的学生姓名

select cid from course WHERE cname in ('物理','体育')	# 先获取到课程的ID 23

select * from score where course_id in (select cid from course WHERE cname in ('物理','体育')) # 在分数表中 获取到选择了课程的id

select score.student_id from score WHERE course_id in (select cid from course WHERE cname in ('物理','体育'))
GROUP BY score.student_id HAVING COUNT(score.course_id) = 1 # 通过分数表 获取到选择课程的学生id 只要课程只出现过一个的ID

select sname from student where sid IN(select score.student_id from score WHERE course_id in	#	通过学生表获取名称
(select cid from course WHERE cname in ('物理','体育')) GROUP BY score.student_id HAVING COUNT(score.course_id) = 1)

5、查询挂科超过两门(包括两门)的学生姓名和班级

select * from score WHERE num < 60	# 先获取分数少于60分不及格的

select student_id from score WHERE num < 60 GROUP BY student_id HAVING COUNT(course_id) >=2 # 按照学生的ID 分组出超过两门或者等于两门的

select * from class INNER JOIN student on class.cid = student.class_id #  链接班级表和学生表

SELECT class.caption, student.sname FROM class INNER JOIN student on class.cid = student.class_id WHERE student.sid in 
(SELECT student_id FROM score WHERE num < 60 GROUP BY student_id HAVING count(course_id) >=2) # 通过拼接的表 拿到学分少于60不及格的班级以及姓名

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