练习答案

-- 2.查询“生物”课程比“物理”课程成绩高的所有学生的学号；

-- select A.student_id,sw,ty from
--  
--         (select student_id,num as sw from score INNER join course on score.course_id = course.cid where course.cname = '生物') as A
--  
--         left join
--  
--         (select student_id,num  as ty from score INNER join course on score.course_id = course.cid where course.cname = '物理') as B
--  
--         on A.student_id = B.student_id where sw > if(isnull(ty),0,ty);
-- 


 
-- 3.查询平均成绩大于60分的同学的学号和平均成绩； 
-- select student_id,avg(number) from l5 group by student_id having avg(number) > 60;


-- 4. 查询所有同学的学号、姓名、选课数、总成绩；

-- SELECT student.sid,student.sname,count(score.student_id),sum(num)
-- from score LEFT JOIN student on score.student_id=student.sid GROUP BY
-- student_id;

-- 5.查询姓“李”的老师的个数；
-- select count(tid) from l3 where tname like "李%";



-- 6.查询没学过“叶平”老师课的同学的学号、姓名；
-- SELECT student.sid,student.sname from student where sid not in ##通过学生表查询没有该课程的学生id和姓名
-- (select student_id from score where course_id in(##成绩表中查出有此课程的学生id
-- SELECT course.cid from course 
-- LEFT JOIN teacher on course.teacher_id=teacher.tid where teacher.tname="李平老师"##课程表和老师表连表，
-- 求出李平老师的课程
-- )
-- GROUP BY student_id)

-- 7.查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
-- SELECT A.student_id,A.sname FROM
-- (SELECT score.student_id,student.sname from score LEFT JOIN student on score.student_id=student.sid where course_id=1) as A,
-- (SELECT score.student_id,student.sname from score LEFT JOIN student on score.student_id=student.sid where course_id=2) as B
-- where A.student_id=B.student_id
-- 
-- 8.查询学过“叶平”老师所教的所有课的同学的学号、姓名；

-- SELECT student.sid,student.sname from student where sid  in ##通过学生表查询有该课程的学生id,姓名
-- (select student_id from score where course_id in(##成绩表中查出有此课程的学生id
-- SELECT course.cid from course 
-- LEFT JOIN teacher on course.teacher_id=teacher.tid where teacher.tname="李平老师"##课程表和老师表连表，
-- -- 求出李平老师的课程
-- )
-- GROUP BY student_id) HAVING count(course_id)=(SELECT count(course.cid) from course 
-- LEFT JOIN teacher on course.teacher_id=teacher.tid where teacher.tname="李平老师")

-- 9.查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；
-- SELECT sid,sname from student INNER JOIN
-- (select student_id,score.num from score where course_id=2) as A on A.student_id=student.sid
-- 
-- INNER JOIN(select student_id,score.num from score where course_id=1) as B on B.student_id=student.sid
-- where A.num<B.num

-- 10.查询有课程成绩小于60分的同学的学号、姓名；
-- select l5.sid,l2.sname from l5,l2 where l5.student_id=l2.sid and l5.number < 60;


-- 11、查询没有学全所有课的同学的学号、姓名；

-- select student_id,sname,course_id from score LEFT JOIN student on score.student_id=student.sid GROUP BY student_id
-- HAVING count(course_id)<(SELECT count(1) from course)




-- 12.查询至少有一门课与学号为“001”的同学所学相同的同学的学号和姓名；
-- SELECT student_id,sname from score LEFT JOIN student on score.student_id=student.sid
-- where student_id !=1 and course_id in
-- (select course_id from score 
-- where student_id=1)
-- GROUP BY student_id;
-- SELECT * from score




-- 13.查询至少学过学号为“001”同学所选课程中任意一门课的其他同学学号和姓名；

-- SELECT student_id,sname from score LEFT JOIN student on score.student_id=student.sid
-- where student_id !=1 and course_id in
-- (select course_id from score 
-- where student_id=1)
-- GROUP BY student_id HAVING count(1)=(select count(course_id) from score 
-- where student_id=1);
-- 




-- 14.查询和“002”号的同学学习的课程完全相同的其他同学学号和姓名；

-- select count(1) from score where student_id = 1;
		-- 

		-- select student_id from score where  student_id in (
		-- 	select student_id from score where student_id !=1 GROUP BY student_id HAVING count(1) = (select count(1) from score where student_id = 1)
		-- ) and course_id in (select course_id from score where student_id = 1) GROUP BY student_id HAVING count(1) = (select count(1) from score where student_id = 1)
		-- 
		-- 
		-- insert into tb(student_id,course_id,num) 
		-- 
		-- select student_id,2,(SELECT AVG(num) from score where course_id = 2) from score where course_id != 2


-- 15、删除学习“叶平”老师课的SC表记录；
-- 
-- delete from score where course_id in (
--         select cid from course left join teacher on course.teacher_id = teacher.tid where teacher.name = '叶平'
--     )



-- 16、向SC表中插入一些记录，这些记录要求符合以下条件：①没有上过编号“002”课程的同学学号；②插入“002”号课程的平均成绩； 
-- 

-- insert into score(student_id, course_id, num) select sid,2,(select avg(num) from score where course_id = 2)
--     from student where sid not in (
--         select student_id from score where course_id = 2
--     )
-- 

-- 17、按平均成绩从低到高显示所有学生的“语文”、“数学”、“英语”三门的课程成绩，按如下形式显示： 学生ID,语文,数学,英语,有效课程数,有效平均分；
-- SELECT 
		-- 	student_id,
		-- 	(select num from score as s2 where s2.student_id=s1.student_id and course_id = 1) as 语文,
		-- 	(select num from score as s2 where s2.student_id=s1.student_id and course_id = 2) as 数学,
		-- 	(select num from score as s2 where s2.student_id=s1.student_id and course_id = 3) as 英语
		-- from score as s1;


-- 18.查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；
-- SELECT course_id,max(num),min(num) FROM score GROUP BY course_id 






-- 19.按各科平均成绩从低到高和及格率的百分数从高到低顺序；
-- SELECT course_id,avg(num) FROM score GROUP BY course_id ORDER BY avg(num)asc 

-- SELECT course_id,avg(num),sum(case when num<60 then 0 else 1 END),sum(1),sum(case when num<60 then 0 else 1 END)/sum(1)as f from score ORDER BY avg(num) ASC,
-- f DESC;
-- SELECT * from score
-- 



-- 20.课程平均分从高到低显示（现实任课老师）；
-- SELECT A.course_id,teacher.tname,A.cc from teacher INNER JOIN 
-- (SELECT teacher_id,course_id,avg(num)as cc from score LEFT JOIN course on score.course_id=course.cid GROUP BY course_id )as A
-- ON teacher.tid=A.teacher_id
-- ORDER BY A.cc desc





-- 21、查询各科成绩前三名的记录:(不考虑成绩并列情况) 
-- 
--  select score.sid,score.course_id,score.num,T.first_num,T.second_num from score left join
--     (
--     select
--         sid,
--         (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 0,1) as first_num,
--         (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 3,1) as second_num
--     from
--         score as s1
--     ) as T
--     on score.sid =T.sid
--     where score.num <= T.first_num and score.num >= T.second_num




-- 22、查询每门课程被选修的学生数；
-- select corse_id,count(student_id) as count from l5 group by corse_id;





-- 23、查询出只选修了一门课程的全部学生的学号和姓名；
-- SELECT student_id,sname,course_id from score LEFT JOIN student on score.student_id=student.sid 
-- 
-- GROUP BY student_id HAVING count(1)=1



-- 24、查询男生、女生的人数；
-- select count(sid),gender from l2 group by gender;
-- 





-- 25、查询姓“张”的学生名单；
-- select * from l2 where sname like "张%";
-- 




-- 26、查询同名同姓学生名单，并统计同名人数；
-- select count(sid),sname from l2 group by sname having count(sid)>1;





-- 27、查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列；
-- select corse_id,cname,avg(number) from l4 left l5 on l4.cid = l5.corse_id order by avg(number) asc; 





-- 28、查询平均成绩大于85的所有学生的学号、姓名和平均成绩；
-- select student_id,sname,avg(number) from l5,l2 group by student_id having avg(number) > 85;
-- 




-- 29、查询课程名称为“数学”，且分数低于60的学生姓名和分数；

-- SELECT student_id,sname,num from score LEFT JOIN student on score.student_id=student.sid where num<60 and course_id in
-- (SELECT cid from course LEFT JOIN score on course.cid=score.course_id where cname="生物" );
-- 
-- SELECT * FROM score


-- 30、查询课程编号为003且课程成绩在80分以上的学生的学号和姓名； 
-- SELECT student_id,sname,num from score LEFT JOIN student on score.student_id=student.sid where course_id=3
-- and num>80;
-- SELECT * from score


-- 31、求选了课程的学生人数

-- SELECT count(course_id) from score GROUP BY student_id;
-- SELECT course_id from score GROUP BY student_id;
-- 
-- SELECT * from score
-- 


-- 32、查询选修“杨艳”老师所授课程的学生中，成绩最高的学生姓名及其成绩；


-- 33、查询各个课程及相应的选修人数；
 
-- select course_id,COUNT(student_id) from score GROUP BY course_id



-- 34、查询不同课程但成绩相同的学生的学号、课程号、学生成绩；

-- select DISTINCT s1.course_id,s2.course_id,s1.num,s2.num from score as s1, score as s2 where s1.num = s2.num and s1.course_id != s2.course_id;



-- 35、查询每门课程成绩最好的前两名；
-- select course_id,MAX(num) from score GROUP BY course_id




-- 36、检索至少选修两门课程的学生学号；

-- SELECT student_id,course_id from score GROUP BY student_id HAVING count(course_id)>=2


-- 37、查询全部学生都选修的课程的课程号和课程名；
-- SELECT student_id,course_id,course.cname from score LEFT JOIN course on score.course_id=course.cid GROUP BY student_id



-- 38、查询没学过“叶平”老师讲授的任一门课程的学生姓名；

--  select student_id,student.sname from score
--     left join student on score.student_id = student.sid
--     where score.course_id not in (
--         select cid from course left join teacher on course.teacher_id = teacher.tid where tname = '张磊老师'
--     )
--     group by student_id


-- 39、查询两门以上不及格课程的同学的学号及其平均成绩；
-- select student_id,avg(num) from score where num<60 GROUP BY student_id HAVING count(1)>=2



-- 40、检索“004”课程分数小于60，按分数降序排列的同学学号；
-- SELECT student_id,course_id,num from score where course_id=4 and num<60 ORDER BY num DESC


-- 41、删除“002”同学的“001”课程的成绩；
-- DELETE from score where student_id=2 and corse_id=1