day 35 作业

班级表：

mysql> create table class(
    -> cid int auto_increment primary key,
    -> caption varchar(32) not null default ''
    -> )charset=utf8;

mysql> insert into class (caption) values ('三年二班'),('一年三班'),('三年一班');

学生表：

create table student(
	sid int auto_increment primary key,
    sname varchar(32) not null default '',
    gender enum('男','女') not null default '男',
    class_id int not null default 0,
    constraint fk_stu_cla foreign key (class_id) references class (cid)
)charset=utf8;

insert into student (sname,gender,class_id) values ('钢弹','女',1),('铁锤','女',1),('山炮','男',2);

老师表：

create table teacher(
	tid int auto_increment primary key,
    tname varchar(32) not null default ''
)charset=utf8;

insert into teacher (tname) values ('波多'),('苍空'),('饭岛');

课程表：

create table course(
	cid int auto_increment primary key,
    cname varchar(32) not null default '',
    teacher_id int not null default 0,
    constraint fk_cou_tea foreign key (teacher_id) references teacher(tid)
)charset=utf8;

insert into course (cname,teacher_id) values ('生物',1),('体育',1),('物理',2);

成绩表

create table score(
	sid int auto_increment primary key,
    student_id int not null default 0,
    course_id int not null default 0,
    number int not null default 0,
    constraint fk_sc_st foreign key (student_id) references student(sid),
    constraint fk_sc_co foreign key (course_id) references course(cid)
)charset=utf8;

insert into score (student_id,course_id,number) values (1,1,60),(1,2,61),(2,2,100);

1. 查询所有大于60分的学生的姓名和学号 (DISTINCT: 去重)

    select distinct student.sid,student.sname from score left join student on score.student_id=student.sid where score.number>=60;
   

-- 2.查询每个老师教授的课程数量 和 老师信息

select count(course.teacher_id) as course_count,tname,tid from course left join teacher on course.teacher_id=teacher.tid group by teacher.tname;

-- 3. 查询学生的信息以及学生所在的班级信息

select * from student left join class on student.class_id=class.cid;

-- 4、学生中男生的个数和女生的个数

select count(gender),gender from student group by gender;

-- 5、获取所有学习'生物'的学生的学号和成绩；姓名

select student.sid,score.number,student.sname from score left join student on score.student_id=student.sid left join course on score.course_id=course.cid where course.cname='生物';

-- 6、查询平均成绩大于60分的同学的学号和平均成绩；

select avg(number),student_id from score group by student_id having avg(number)>60;

-- 7、查询姓“李”的老师的个数；

select count(tname) from teacher group by tname having tname like '李%';

-- 8、查询课程成绩小于60分的同学的学号、姓名；

select student.sid,student.sname from score left join student on score.student_id=student.sid where number<60;

-- 9. 删除学习“叶平”老师课的SC表记录

delete from score where course_id in (select course.cid from course left join teacher on course.teacher_id=teacher.tid where teacher.tname='李小龙')；

-- 10.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(number),min(number) from score group by course_id;

-- 11.查询每门课程被选修的学生数

select count(course_id),course_id from score group by course_id;

-- 12.查询姓“张”的学生名单；

select * from student where sname like '张%';

-- 13.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 sid,avg(number) from score group by course_id order by avg(number) asc,sid desc;

-- 14.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.sid,student.sname,avg(number) from score left join student on score.student_id=student.sid group by score.student_id having avg(number)>85;

-- 15.查询课程编号为3且课程成绩在80分以上的学生的学号和姓名；

select student.sid,student.sname from score left join student on score.student_id=student.sid where score.course_id=3 and number>80;

-- 16.查询各个课程及相应的选修人数

select count(course_id),course_id from score group by course_id;

-- 17.查询“4”课程分数小于60，按分数降序排列的同学学号

select student_id from score where course_id=4 order by student_id desc;

-- 18.删除学号为“2”的同学的“1”课程的成绩

delete from score where student_id=2 and course_id=1;