4.2 - MySQL

一、表关系
    请创建如下表，并创建相关约束

        班级表：class                           　　　 学生表：student
        cid    caption    grade_id                　　　sid    sname    gender    class_id
        1     一年一班         1                    　　　1     乔丹      　女          1
        2     二年一班         2                    　　　2    艾弗森       女          1
        3     三年二班         3                    　　　3     科比        男          2

        老师表：teacher                             　　课程表：course
        tid    tname                             　　　　cid    cname    teacher_id
        1     张三                                　　　　1     生物          1
        2     李四                                　　　　2     体育          1
        3     王五                                　　　　3     物理          2

        成绩表：score                            　　　　年级表：class_grade
        sid    student_id    course_id    score       　　gid    gname
        1         1           1        　　60          　　1     一年级
        2         1           2        　　59          　　2     二年级
        3         2           2        　　99          　　3     三年级

        班级任职表：teach2cls
        tcid    tid    cid
        1      1      1
        2      1      2
        3      2      1
        4      3      2

二、操作表
    # 题目：
        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
);

准备数据

insert into class_grade(gname) values    # 4个年级
('一年级'),
('二年级'),
('三年级'),
('四年级');

insert into class(caption,grade_id) values  # 9个班级
('一年一班',1),
('一年二班',1),
('一年三班',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);

insert into student(sname,gender,class_id) values  # 12个学生
('乔丹','女',1),
('艾弗森','女',1),
('科比','男',2),
('alice','女',2),
('alex','男',3),
('egon','男',4),
('lily','女',5),
('lucy','女',6),
('json','男',6),
('哈妹','女',7),
('霍建华','男',8),
('林心如','女',9);

insert into teacher(tname) values   # 4个老师
('张三'),
('李四'),
('王五'),
('赵六');

insert into course(cname,teacher_id) values  # 9个课程
('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);

insert into score(student_id,course_id,score) values    # 12个学生，9个课程
(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),
(5,1,59),
(5,2,33),
(5,3,12),
(5,4,88),
(6,5,100),
(6,6,60),
(6,7,59),
(6,8,100),
(7,9,20),
(7,1,36),
(7,3,57),
(7,5,60),
(8,2,61),
(8,4,59),
(8,6,62),
(9,8,59),
(9,1,60),
(9,2,61),
(9,3,21),
(10,5,68),
(10,9,99),
(11,1,89),
(11,7,99),
(12,3,100),
(12,8,60);

insert into teach2cls(tid,cid) values   # 4个老师 9个班级
(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);

补充数据

insert into score(student_id,course_id,score) values    # 12个学生，9个课程
(6,1,60),
(6,3,99),
(10,1,70),
(10,3,88);

insert into student(sname,gender,class_id) values  # 12个学生
('alice_1','女',1),
('alice_2','女',2);

insert into teacher(tname) values   # 4个老师
('李杰');

insert into class(caption,grade_id) values  # 9个班级
('一年四班',1),
('一年五班',1);

insert into class_grade(gname) values    # 4个年级
('五年级');
insert into class(caption,grade_id) values  # 9个班级
('五年一班',5);

insert into course(cname,teacher_id) values  # 9个课程
('研究',1);

insert into teach2cls(tid,cid) values   # 4个老师 9个班级
(1,12),
(2,12);

insert into score(student_id,course_id,score) values    # 12个学生，9个课程
(1,2,60),
(2,2,59);

insert into student(sname,gender,class_id) values  # 12个学生
('张一','女',1),
('张二','男',2);

insert into student(sname,gender,class_id) values  # 12个学生
('张一','女',3),
('张二','男',4);

insert into score(student_id,course_id,score) values    # 12个学生，9个课程
(7,8,60),
(7,10,60),
(4,10,88),
(6,9,88);

insert into score(student_id,course_id,score) values    # 12个学生，9个课程
(2,1,60);

insert into score(student_id,course_id,score) values    # 12个学生，9个课程
(1,1,60),
(2,1,70),
(3,1,80);

http://www.cnblogs.com/wupeiqi/articles/5748496.html
答案：

        #1、自行创建测试数据；
            见table_data.txt

        #2、查询学生总人数；
            select
                count(sid) as count_student
            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                        # 没有包含班级数相同得排名前三
                class_grade.gname
            from
                class_grade
            inner join (
                select
                    grade_id,
                    count(cid) as count_cid
                from
                    class
                group by
                    grade_id
                order by
                    count_cid desc
                limit 3
            ) as t1 on class_grade.gid = t1.grade_id;

            select                       # 包含了班级数相同得排名前三
                gname
            from
                class_grade
            where
                gid in (
                    select
                        grade_id
                    from
                        class
                    group by
                        grade_id
                    having
                        count(cid) in (
                            select            # 子查询in()里面不能包含limit; This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
                                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              # 包含了平均成绩相等得情况下，多个学生
                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,
                        class
                    where
                        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        # 包含了没有选课得学生left join
                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           # 包含了成绩最高，最低相等得情况
                student.sname,
                course.cname,
                t1.score
            from
                (
                    select
                        student_id,
                        course_id,
                        score
                    from
                        score
                    where
                        student_id = 2
                        and score in (
                            (
                                select
                                    max(score)
                                from
                                    score
                                where
                                    student_id = 2

                            ),
                            (
                                select
                                    min(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          # 包含了姓"李"的老师没有带班级
                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

            select
                class.cid,
                class.caption,
                class_grade.gname,
                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
                                t1.cid
                            from
                                (
                                    select
                                        class.cid,
                                        class.caption,
                                        class_grade.gname,
                                        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 grade_layer
                                    from
                                        class,
                                        class_grade
                                    where
                                        class.grade_id = class_grade.gid
                                ) as t1
                            where
                                t1.grade_layer = '高'
                        )
                );

                # 另一种方法：
                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
                    where
                        course_id in (
                            select
                                cid
                            from
                                course
                            inner join teacher on teacher.tid = course.teacher_id
                            where teacher.tname = '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
                        tid
                    from
                        teach2cls
                    group by
                        tid
                    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；
            select      # 包含了班级数最多相等的情况
                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  # 包含了学号为1的同学
                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   # 没有包含学号为1的同学
                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；
            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”、“音乐”三门的课程成绩，按如下形式显示： 学生ID,语文,数学,音乐,有效课程数,有效平均分； # ****
            select
                sc.student_id,
                (select score.score from score left join course on score.course_id = course.cid where course.cname = '语文' and score.student_id = sc.student_id) as yw,
                (select score.score from score left join course on score.course_id = course.cid where course.cname = '数学' and score.student_id = sc.student_id) as sx,
                (select score.score from score left join course on score.course_id = course.cid where course.cname = '音乐' and score.student_id = sc.student_id) as yy,
                count(sc.course_id),
                avg(sc.score)
            from
                score as sc
            group by
                sc.student_id
            order by
                avg(sc.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,
                max(score.score) as max_score,
                min(score.score) as min_score
            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 percent
            from
                score
            group by
                course_id
            order by
                avg(score) asc,
                percent desc;

        #30、课程平均分从高到低显示（显示任课老师）；
            select
                t1.course_id,
                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)  # 和44题相似！！
            select
                score.sid,
                score.student_id,
                score.course_id,
                score.score,
                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；
            select      # 包含了没有被选修的课程显示0
                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；
            select         # 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
                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；
            select             # 包含了没有被选修的课程显示0
                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；
            select distinct              # 不同的学生之间，成绩相同 这里有问题！！
                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；  #??? 这里有个问题,成绩相同的学生，会都录入！！
            select
                student.sid,
                student.sname,
                t2.course_id,
                t2.score,
                t2.first_score,
                t2.second_score
            from
                student
            inner join (                    # 这里会出现成绩相同的情况，录入了超过了两名的学生！！
                select
                    score.student_id,
                    score.course_id,
                    score.score,
                    t1.first_score,
                    t1.second_score
                from
                    score
                inner join (
                    select
                        s1.sid,
                        (select s2.score from score as s2 where s1.course_id = s2.course_id order by s2.score desc limit 0,1) as first_score,
                        (select s3.score from score as s3 where s1.course_id = s3.course_id order by s3.score desc limit 1,1) as second_score
                    from
                        score as s1
                ) as t1 on score.sid = t1.sid
                where
                    score.score in (           # ？？在这，会超过两个！！
                        t1.first_score,
                        t1.second_score
                    )
            ) as t2 on student.sid = t2.student_id;

        **************************************************************
            # 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      # 需要在包一层select(),否则 You can't specify target table 'score' for update in FROM clause
                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
                );