MySQL查询练习

表关系

查询练习

1、自行创建测试数据；
2、查询学生总人数；
3、查询“生物”课程和“物理”课程成绩都及格的学生id和姓名；
4、查询每个年级的班级数，取出班级数最多的前三个年级；
5、查询平均成绩最高和最低的学生的id和姓名以及平均成绩；
6、查询每个年级的学生人数；
7、查询每位学生的学号，姓名，选课数，平均成绩；
8、查询学生编号为“2”的学生的姓名、该学生成绩最高的课程名、成绩最低的课程名及分数；
9、查询姓“李”的老师的个数和所带班级数；
10、查询班级数小于5的年级id和年级名；
11、查询班级信息，包括班级id、班级名称、年级、年级级别(12为低年级，34为中年级，56为高年级)，示例结果如下；
    班级id   班级名称   年级 年级级别
    1  一年一班   一年级    低
12、查询学过“张三”老师2门课以上的同学的学号、姓名；
13、查询教授课程超过2门的老师的id和姓名；
14、查询学过编号“1”课程和编号“2”课程的同学的学号、姓名；
15、查询没有带过高年级的老师id和姓名；
16、查询学过“张三”老师所教的所有课的同学的学号、姓名；
17、查询带过超过2个班级的老师的id和姓名；
18、查询课程编号“2”的成绩比课程编号“1”课程低的所有同学的学号、姓名；
19、查询所带班级数最多的老师id和姓名；
20、查询有课程成绩小于60分的同学的学号、姓名；
21、查询没有学全所有课的同学的学号、姓名；
22、查询至少有一门课与学号为“1”的同学所学相同的同学的学号和姓名；
23、查询至少学过学号为“1”同学所选课程中任意一门课的其他同学学号和姓名；
24、查询和“2”号同学学习的课程完全相同的其他同学的学号和姓名；
25、删除学习“张三”老师课的score表记录；
26、向score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“2”课程的同学学号；②插入“2”号课程的平均成绩；
27、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；
28、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；
29、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；
30、课程平均分从高到低显示（显示任课老师）；
31、查询各科成绩前三名的记录(不考虑成绩并列情况)
32、查询每门课程被选修的学生数；
33、查询选修了2门以上课程的全部学生的学号和姓名；
34、查询男生、女生的人数，按倒序排列；
35、查询姓“张”的学生名单；
36、查询同名同姓学生名单，并统计同名人数；
37、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；
38、查询课程名称为“数学”，且分数低于60的学生姓名和分数；
39、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；
40、求选修了课程的学生人数
41、查询选修“王五”老师所授课程的学生中，成绩最高和最低的学生姓名及其成绩；
42、查询各个课程及相应的选修人数；
43、查询不同课程但成绩相同的学生的学号、课程号、学生成绩；
44、查询每门课程成绩最好的前两名学生id和姓名；
45、检索至少选修两门课程的学生学号；
46、查询没有学生选修的课程的课程号和课程名；
47、查询没带过任何班级的老师id和姓名；
48、查询有两门以上课程超过80分的学生id及其平均成绩；
49、检索“3”课程分数小于60，按分数降序排列的同学学号；
50、删除编号为“2”的同学的“1”课程的成绩；
51、查询同时选修了物理课和生物课的学生id和姓名；

建立相关表

create table class_grade(
    gid int primary key auto_increment,
    gname varchar(16) not null unique
);

create table class(
    cid int primary key auto_increment,
    caption varchar(16) not null,
    grade_id int not null,
    foreign key(grade_id) references class_grade(gid)
);

create table student(
    sid int primary key auto_increment,
    sname varchar(16) not null,
    gender enum('女','男') not null default '女',
    class_id int not null,
    foreign key(class_id) references class(cid)
);

create table teacher(
    tid int primary key auto_increment,
    tname varchar(16) not null
);

create table course(
    cid int primary key auto_increment,
    cname varchar(16) not null,
    teacher_id int not null,
    foreign key(teacher_id) references teacher(tid)
);

create table score(
    sid int not null unique auto_increment,
    student_id int not null,
    course_id int not null,
    score int not null,
    primary key(student_id,course_id),
    foreign key(student_id) references student(sid)
    on delete cascade
    on update cascade,
    foreign key(course_id) references course(cid)
    on delete cascade
    on update cascade
);

create table teach2cls(
    tcid int not null unique auto_increment,
    tid int not null,
    cid int not null,
    primary key(tid,cid),
    foreign key(tid) references teacher(tid)
    on delete cascade
    on update cascade,
    foreign key(cid) references class(cid)
    on delete cascade
    on update cascade
);

建表

添加测试数据

# 5 个年级
insert into class_grade(gname) values
('一年级'),
('二年级'),
('三年级'),
('四年级'),
('五年级');


# 5 个老师
insert into teacher(tname) values
('张三'),
('李四'),
('王五'),
('李杰'),
('赵六');

# 12 个班级
insert into class(caption,grade_id) values
('一年一班',1),
('一年二班',1),
('一年三班',1),
('一年四班',1),
('一年五班',1),
('二年一班',2),
('二年二班',2),
('三年一班',3),
('三年二班',3),
('四年一班',4),
('四年二班',4),
('五年一班',5);

# 10 个课程
insert into course(cname,teacher_id) values
('生物',1),
('体育',1),
('物理',2),
('化学',3),
('美术',4),
('音乐',2),
('语文',3),
('数学',4),
('地理',2),
('研究',1);

# 18 个学生
insert into student(sname,gender,class_id) values  # 12个学生
('孙尚香','女',1),
('貂蝉','女',1),
('刘备','男',2),
('孙二娘','女',2),
('张飞','男',3),
('关羽','男',4),
('林黛玉','女',5),
('薛宝钗','女',6),
('宋江','男',6),
('白骨精','女',7),
('猪八戒','男',8),
('王熙凤','女',1),
('李师师','女',2),
('金翠莲','女',9),
('如花','女',1),
('沙僧','男',2),
('李美丽','女',3),
('金角大王','男',4);


insert into score(student_id,course_id,score) values
(1,1,60),
(1,2,59),
(1,3,58),
(1,4,22),
(1,5,59),
(1,6,60),
(1,7,99),
(1,8,100),
(1,9,88),
(2,1,99),
(2,2,99),
(2,3,89),
(2,4,60),
(2,5,59),
(2,6,33),
(2,7,56),
(2,8,59),
(2,9,60),
(3,1,59),
(3,3,30),
(3,5,28),
(3,7,70),
(3,9,60),
(4,2,59),
(4,4,100),
(4,6,90),
(4,8,80),
(4,10,88),
(5,1,59),
(5,2,33),
(5,3,12),
(5,4,88),
(6,1,60),
(6,3,99),
(6,5,100),
(6,6,60),
(6,7,59),
(6,8,100),
(6,9,88),
(7,9,20),
(7,1,36),
(7,3,57),
(7,5,60),
(7,8,60),
(7,10,60),
(8,2,61),
(8,4,59),
(8,6,62),
(9,8,59),
(9,1,60),
(9,2,61),
(9,3,21),
(10,1,70),
(10,3,88),
(10,5,68),
(10,9,99),
(11,1,89),
(11,7,99),
(12,3,100),
(12,8,60);


insert into teach2cls(tid,cid) values
(1,1),
(1,2),
(1,3),
(1,5),
(2,4),
(2,6),
(2,8),
(2,9),
(2,1),
(2,5),
(3,7),
(3,1),
(3,3),
(3,5),
(3,9),
(4,7),
(4,2),
(4,4),
(4,6),
(4,8),
(4,1),
(1,12),
(2,12);

参考SQL

1、自行创建测试数据；
详见"db5_结构+数据.sql"
2、查询学生总人数；
select count(sid) from student ;
3、查询“生物”课程和“物理”课程成绩都及格的学生id和姓名；
select
  sid,
  sname
from
  student
where
  sid in(
    select
      score.student_id
    from
      score
    inner join course on score.course_id=course.cid
    where
      course.cname in(
        '生物',
        '物理'
        )
        and score.score >= 60
        group by
            score.student_id
        having
            count(course_id)=2
      );


4、查询每个年级的班级数，取出班级数最多的前三个年级；

select
    gname
from
    class_grade
where
    gid in (
        select
            grade_id
        from
            class
        group by
            grade_id
        having
            count(cid) in (
                select
                    t1.count_cid
                from
                    (
                        select distinct
                            count(cid) as count_cid
                        from
                            class
                        group by
                            grade_id
                        order by
                            count_cid desc
                        limit 3
                    ) as t1
                )
            );


5、查询平均成绩最高和最低的学生的id和姓名以及平均成绩；

select
    student.sid,
    student.sname,
    t1.avg_score
from
    student
inner join (
    select
        student_id,
        avg(score) as avg_score
    from
        score
    group by
        student_id
    having
        avg(score) in (
            (
                select
                    avg(score) as max_avg_score
                from
                    score
                group by
                    student_id
                order by
                    avg(score) desc
                limit 1
            ),
            (
                select
                    avg(score) as min_avg_score
                from
                    score
                group by
                    student_id
                order by
                    avg(score) asc
                limit 1
            )
        )
) as t1 on student.sid = t1.student_id;

6、查询每个年级的学生人数；
select
    t1.grade_id,
    count(t1.sid) as count_student
from
    (
        select
            student.sid,
            class.grade_id
        from
            student
        inner join class on student.class_id=class.cid
    ) as t1
group by
    t1.grade_id;

7、查询每位学生的学号，姓名，选课数，平均成绩；
select
    student.sid,
    student.sname,
    t1.count_course,
    t1.avg_score
from
    student
left join (
    select
        student_id,
        count(course_id) as count_course,
        avg(score) as avg_score
    from
        score
    group by
        student_id
) as t1 on student.sid = t1.student_id;


8、查询学生编号为“2”的学生的姓名、该学生成绩最高的课程名、成绩最低的课程名及分数；

select
    student.sname,
    course.cname,
    t1.score
from(

        select
            student_id,
            course_id,
            score
        from
            score
        where
            student_id=2
            and score in(
                (

                    select
                        min(score)
                    from
                        score
                    where
                        student_id=2
                ),
                (
                    select
                        max(score)
                    from
                        score
                    where
                        student_id=2
                )
            )
    ) as t1
    inner join student on t1.student_id=student.sid
    inner join course on t1.course_id=course.cid;

9、查询姓“李”的老师的个数和所带班级数；

select
    teacher.tid,
    teacher.tname,
    t1.count_cid
from
    teacher
left join (
    select
        tid,
        count(cid) as count_cid
    from
        teach2cls
    where
        tid in (
            select
                tid
            from
                teacher
            where
                tname like '李%'
        )
    group by
        tid
) as t1 on teacher.tid = t1.tid
where
    teacher.tname like '李%';


10、查询班级数小于5的年级id和年级名；
select
    gid,
    gname
from
    class_grade
where
    gid in (
        select
            grade_id
        from
            class
        group by
            grade_id
        having
            count(caption)<5
    );


11、查询班级信息，包括班级id、班级名称、年级、年级级别(12为低年级，34为中年级，56为高年级)，示例结果如下；
    班级id   班级名称   年级 年级级别
    1  一年一班   一年级    低
# case when ... then

select
    class.cid 班级id,
    class.caption 班级名称,
    class_grade.gname 年级,
    case when class_grade.gid between 1 and 2  then '低'
    when class_grade.gid between 3 and 4  then '中'
    when class_grade.gid between 5 and 6  then '高' else 0 end as '年级级别'
from
    class,
    class_grade
where
    class.grade_id = class_grade.gid;


12、查询学过“张三”老师2门课以上的同学的学号、姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select
            student_id
        from
            score
        where
            course_id in (
                select
                    course.cid
                from
                    teacher,
                    course
                where
                    teacher.tid = course.teacher_id
                    and teacher.tname = '张三'
            )
        group by
            student_id
        having
            count(course_id) > 2
    );

13、查询教授课程超过2门的老师的id和姓名；

select
    tid,
    tname
from
    teacher
where
    tid in (
        select
            teacher_id
        from
            course
        group by
            teacher_id
        having
            count(cid) > 2
    );

14、查询学过编号“1”课程和编号“2”课程的同学的学号、姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select distinct
            student_id
        from
            score
        where
            course_id in (
                1,
                2
            )
    );

15、查询没有带过高年级的老师id和姓名；

select
    tid,
    tname
from
    teacher
where
    tid not in (
        select
            tid
        from
            teach2cls
        where
            cid in (
                select
                    cid
                from
                    class
                where
                    grade_id in (
                        5,
                        6
                    )
            )
    );

16、查询学过“张三”老师所教的所有课的同学的学号、姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select
            student_id
        from
            score
        inner join course on score.course_id=course.cid
        where teacher_id in(
            select
                sid
            from
                teacher
            where
                sname='张三'

        )


    )

17、查询带过超过2个班级的老师的id和姓名；

select
    tid,
    tname
from
    teacher
where
    tid in (
        select
            teacher_id
        from
            course
        group by
            teacher_id
        having
            count(cid) > 2
    );

18、查询课程编号“2”的成绩比课程编号“1”课程低的所有同学的学号、姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select
            t1.student_id
        from
            (
                select
                    student_id,
                    score
                from
                    score
                where
                    course_id = 2
                group by
                    student_id
            ) as t1,
            (
                select
                    student_id,
                    score
                from
                    score
                where
                    course_id = 1
                group by
                    student_id
            ) as t2
        where
            t1.student_id = t2.student_id
            and t1.score < t2.score
    );

19、查询所带班级数最多的老师id和姓名；

# 考虑班级数并列最多的情况

select
    tid,
    tname
from
    teacher
where
    tid in (
        select
            tid
        from
            teach2cls
        group by
            tid
        having
            count(cid) = (
                select
                    count(cid)
                from
                    teach2cls
                group by
                    tid
                order by
                    count(cid) desc
                limit 1
            )
    );

20、查询有课程成绩小于60分的同学的学号、姓名；
select
    sid,
    sname
from
    student
where
    sid in (
        select distinct
            student_id
        from
            score
        where
            score < 60
    );

21、查询没有学全所有课的同学的学号、姓名；
select
    sid,
    sname
from
    student
where
    sid not in (
        select
            student_id
        from
            score
        group by
            student_id
        having
            count(course_id) = (
                select
                    count(cid)
                from
                    course
            )
    );
22、查询至少有一门课与学号为“1”的同学所学相同的同学的学号和姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select
            student_id
        from
            score
        where
            course_id in (
                select
                    course_id
                from
                    score
                where
                    student_id = 1
            )
        group by
            student_id
    );

23、查询至少学过学号为“1”同学所选课程中任意一门课的其他同学学号和姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select
            student_id
        from
            score
        where
            course_id in (
                select
                    course_id
                from
                    score
                where
                    student_id = 1
            )
        group by
            student_id
    )
    and sid != 1;

24、查询和“2”号同学学习的课程完全相同的其他同学的学号和姓名；
select
    sid,
    sname
from
    student
where
    sid in (
        select
            score.student_id
        from
            score,
            (
                select
                    course_id
                from
                    score
                where
                    student_id = 2
            ) as t1
        where
            score.course_id = t1.course_id
            and score.student_id != 2
        group by
            score.student_id
        having
            count(score.course_id) = (
                select
                    count(course_id)
                from
                    score
                where
                    student_id = 2
            )
    );


25、删除学习“张三”老师课的score表记录；
delete from
    score
where
    course_id in (
        select
            course.cid
        from
            course,
            teacher
        where
            course.teacher_id = teacher.tid
            and teacher.tname = '张三'
    );

26、向score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“2”课程的同学学号；②插入“2”号课程的平均成绩；

# 实际结果会和上面一题有冲突，张三老师所教的2号课程

insert into score(student_id,course_id,score)
select
    t1.sid,
    2,
    t2.avg_score
from
    (
        select
            sid
        from
            student
        where
            sid not in (
                select
                    student_id
                from
                    score
                where
                    course_id = 2
            )
    ) as t1,
    (
        select
            avg(score) as avg_score
        from
            score
        where
            course_id = 2
    ) as t2;

27、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；

# 有效成绩含所有课程，包括语数英

select
    t1.student_id as 学生ID,
    (select score.score from score left join course on score.course_id = course.cid where course.cname = '语文' and score.student_id = t1.student_id) as 语文,
    (select score.score from score left join course on score.course_id = course.cid where course.cname = '数学' and score.student_id = t1.student_id) as 数学,
    (select score.score from score left join course on score.course_id = course.cid where course.cname = '音乐' and score.student_id = t1.student_id) as 英语,
    count(t1.course_id) as 有效课程数,
    avg(t1.score) as 有效平均分
from
    score as t1
group by
    t1.student_id
order by
    avg(t1.score) asc;

28、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.cid as 课程ID,
    max(score.score) as 最高分,
    min(score.score) as 最低分
from
    course
left join score on course.cid = score.course_id
group by
    score.course_id;

29、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(score) as avg_score,
    sum(case when score.score > 60 then 1 else 0 end) / count(1) * 100 as 及格率
from
    score
group by
    course_id
order by
    avg(score) desc,
    及格率 asc;

30、课程平均分从高到低显示（显示任课老师）；
select
    course.cname,
    t1.avg_score,
    teacher.tname
from
    course,
    teacher,
    (
        select
            course_id,
            avg(score) as avg_score
        from
            score
        group by
            course_id
        order by
            avg_score desc
    ) as t1
where
    course.cid = t1.course_id
    and course.teacher_id = teacher.tid
order by
    t1.avg_score desc;

31、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.student_id,
    score.course_id,
    score.sid,
    t1.first_score,
    t1.second_score,
    t1.third_score
from
    score
inner join (
    select
        s1.sid,
        (select score from score as s2 where s1.course_id = s2.course_id order by score desc limit 0,1) as first_score,
        (select score from score as s3 where s1.course_id = s3.course_id order by score desc limit 1,1) as second_score,
        (select score from score as s4 where s1.course_id = s4.course_id order by score desc limit 2,1) as third_score
    from
        score as s1
) as t1 on score.sid = t1.sid
where
    score.score in (
        t1.first_score,
        t1.second_score,
        t1.third_score
    );

32、查询每门课程被选修的学生数；

# 包含了没有被选修的课程显示0
select
    course.cid,
    ifnull(t1.count_students,0) as count_student
from
    course
left join (
    select
        course_id,
        count(student_id) as count_students
    from
        score
    group by
        course_id
) as t1 on course.cid = t1.course_id;

33、查询选修了2门以上课程的全部学生的学号和姓名；

select
    sid,
    sname
from
    student
where
    sid in (
        select
            student_id
        from
            score
        group by
            student_id
        having
            count(course_id) > 2
    );

34、查询男生、女生的人数，按倒序排列；

select
    gender,
    count(sid) as count_student
from
    student
group by
    gender
order by
    count_student desc;

35、查询姓“张”的学生名单；

select
    student.sid,
    student.sname,
    student.gender,
    class.caption
from
    student
inner join class on student.class_id = class.cid
where
    student.sname like '张%';

36、查询同名同姓学生名单，并统计同名人数；
select
    sname,
    count(sname) as count_sname
from
    student
group by
    sname
having
    count(sname) > 1;

37、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
    course_id,
    avg(score) as avg_score
from
    score
group by
    course_id
order by
    avg_score,
    course_id desc;

38、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.sname,
    t1.score
from
    student
inner join (
    select
        score.student_id,
        score.score
    from
        score
    inner join course on score.course_id = course.cid
    where
        course.cname = '数学'
        and score.score < 60
) as t1 on student.sid = t1.student_id;
39、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
    sid,
    sname
from
    student
where sid in (
    select
        student_id
    from
        score
    where
        course_id = 3
        and score > 80
);
40、求选修了课程的学生人数
select
    course_id,
    count(student_id) as count_student
from
    score
group by
    course_id;

41、查询选修“王五”老师所授课程的学生中，成绩最高和最低的学生姓名及其成绩；
# 教的课可能包含多门，按课程id来分的！包含了最高，最低成绩相等的情况

select
    student.sname,
    t2.course_id,
    t2.score,
    t2.max_score,
    t2.min_score
from
    student
inner join (
    select
        score.student_id,
        score.course_id,
        score.score,
        t1.max_score,
        t1.min_score
    from
        score,
        (
            select
                course_id,
                max(score) as max_score ,
                min(score) as min_score
            from
                score
            where
                course_id in (
                    select
                        cid
                    from
                        course
                    inner join teacher on course.teacher_id = teacher.tid
                    where
                        teacher.tname = '王五'
                )
            group by
                course_id
        ) as t1
    where
        score.course_id = t1.course_id
        and score.score in (
            max_score,
            min_score
        )
) as t2 on student.sid = t2.student_id;

42、查询各个课程及相应的选修人数；
# 包含了没有被选修的课程显示0

select
    course.cname,
    ifnull(t1.count_student,0) as count_student
from
    course
left join (
    select
        course_id,
        count(student_id) as count_student
    from
        score
    group by
        course_id
) as t1 on course.cid = t1.course_id;
43、查询不同课程但成绩相同的学生的学号、课程号、学生成绩；
# 不同的学生之间，成绩相同 这里有问题！！
select distinct
    s1.student_id,
    s2.student_id,
    s1.course_id as s1_course_id,
    s2.course_id as s2_course_id,
    s1.score,
    s2.score
from
    score as s1,
    score as s2
where
    s1.course_id != s2.course_id
    and s1.score = s2.score;

select distinct             # 同一个学生，成绩相同
    s1.student_id,
    s2.student_id,
    s1.course_id as s1_course_id,
    s2.course_id as s2_course_id,
    s1.score,
    s2.score
from
    score as s1,
    score as s2
where
    s1.student_id = s2.student_id
    and s1.course_id != s2.course_id
    and s1.score = s2.score;
44、查询每门课程成绩最好的前两名学生id和姓名；

# 这个有排名 包括了成绩相同的！
select
    c.sname,
    d.cname,
    a.score
from
    score a
inner join (
    select course_id, score, rank
        from (
             select a.course_id, a.score, count(*) as rank
                from (select course_id, score
                       from score
                        group by course_id, score
                        order by course_id, score desc) a
                inner join (select course_id, score
                            from score
                            group by course_id, score
                            order by course_id, score desc) b
                on a.course_id = b.course_id
                    and a.score <= b.score
             group by course_id, score
        ) t1
        where rank in (1, 2)
        order by course_id, rank
   ) b
on a.course_id = b.course_id
    and a.score = b.score
inner join student c
    on a.student_id = c.sid
inner join course d
    on a.course_id = d.cid;

45、检索至少选修两门课程的学生学号；
select
    student_id
from
    score
group by
    student_id
having
    count(course_id) >= 2;
46、查询没有学生选修的课程的课程号和课程名；

select
    cid,
    cname
from
    course
where
    cid not in (
        select
            course_id
        from
            score
        group by
            course_id
    );

47、查询没带过任何班级的老师id和姓名；
select
    tid,
    tname
from
    teacher
where
    tid not in (
        select
            tid
        from
            teach2cls
        group by
            tid
    );

48、查询有两门以上课程超过80分的学生id及其平均成绩；
select
    student_id,
    avg(score) as avg_score
from
    score
where
    student_id in (
        select
            student_id
        from
            score
        where
            score > 80
        group by
            student_id
        having
            count(course_id) > 2
    )
group by
    student_id;

49、检索“3”课程分数小于60，按分数降序排列的同学学号；
select
    student_id,
    score
from
    score
where
    course_id = 3
    and score < 60
order by
    score desc;

50、删除编号为“2”的同学的“1”课程的成绩；
delete from
    score
where
    sid = (
        select
            t1.sid
        from
            (
                select
                    sid
                from
                    score
                where
                    student_id = 2
                    and course_id = 1
            ) as t1
    );

51、查询同时选修了物理课和生物课的学生id和姓名；
select
    sid,
    sname
from
    student
where
    sid in (
        select
            student_id
        from
            score
        where
            course_id in (
                select
                    cid
                from
                    course
                where
                    course.cname in (
                        '物理',
                        '生物'
                    )
            )
        group by
            student_id
        having
            count(course_id) = 2
    );