mysql之连接查询小作业

#数据准备
drop table if exists class;
create table class(
    class_no int(2) unsigned zerofill primary key auto_increment comment '班级编号',
    class_name varchar(30) not null comment '班级名称'
);
insert into class values(1, '培优班');
insert into class values(2, '普通班');
insert into class values(3, '进阶班');

drop table if exists student;
create table student(
    stu_no int(2) unsigned zerofill primary key auto_increment comment '学员编号',
    stu_name varchar(30) not null comment '学员姓名',
    stu_sex varchar(3) not null comment '学员性别',
    stu_age tinyint(2) unsigned zerofill comment '学员年代',
    grade double(5,2) zerofill comment '成绩',
    class_no int(2) unsigned zerofill comment '所在班级编号',
    foreign key(class_no) references class(class_no)
);
insert into student values(01, '李白', '男', 18, 60, 01);
insert into student values(02, '杜甫', '男', 20, 76, 01);
insert into student values(03, '张飞', '男', 32, 80, 02);
insert into student values(04, '韩信', '男', 26, 98, 02);
insert into student values(05, '了龙', '男', 27, 56, 02);
insert into student values(06, '大乔', '女', 17, 88, 01);
insert into student values(07, '小乔', '女', 16, 96, 01);
insert into student values(08, '小乔', '女', 16, 90, 01);
insert into student values(09, '关哥', '男', 32, 80, 02);
insert into student values(10, '刘备', '男', 36, 98, null);
alter table student drop foreign key `student_ibfk_1`;
*********************************************************************************************************************************************

1: 查询出‘培优班’的学员
  // 子查询
  select * from student where class_no = (select class_no from class where class_name = "培优班");
  // 内连接
  select * from student inner join class on student.class_no = class.class_no and class_name = "培优班";
  // 自然连接
  select * from student natural join class where class_name = "培优班";

2: 查询出‘普通班’成绩高于85分学员
  select * from student where class_no = (select class_no from class where class_name = "普通班") and grade > 85;
  select * from student inner join class on student.class_no = class.class_no and class_name = "普通班" and grade > 85;
  select * from student natural join class where class_name = "普通班" and grade > 85;

3: 写出一个迪卡尔集的查询结果
  select * from student cross join class;
  select * from student inner join class;

4: 查询出每一个班级的平均分
  // 包含班级号为null的结果
  select class_no,avg(grade) from student group by class_no;
  // 不包含班级号为null的结果
  select class_no,avg(grade) from student inner join class using(class_no) group by class_no; // 不包括class_no为null的结果

5: 查询出每一个学员的姓名和所在的班级名称
  select stu_name,class_name from student inner join class using(class_no);
  select stu_name,class_name from student inner join class on student.class_no=class.class_no;
  select class_name,stu_name from student natural join class;

6: 查询出培优班的最低分是多少
  select min(grade) from student where class_no = (select class_no from class where class_name = "培优班");
  select min(grade) from student inner join class on class.class_no = student.class_no and class_name = "培优班";
  select min(grade) from student natural join class where class_name = "培优班";

7: 查询出培优班成绩最差的学员信息(成绩最差的不一定是一个人)
  select * from student where class_no = (select class_no from class where class_name = "培优班") and grade = (select min(grade) from student where class_no = (select class_no from class where class_name = "培优班"));
  select * from student where (class_no,grade) = (select class_no,min(grade) from student natural join class where class_name = "培优班");

8: 查询出普通班成绩最好的学员信息
  select * from student natural join class where class_name = "普通班" order by grade desc limit 1;
  (改下第七题的条件就好)

9: 查询出成绩最好的学员的姓名 以及 他们的班级名称
  // 结果为多条记录的查询
  select stu_name,class_name from student natural left join class where grade = (select max(grade) from student);

10: 查询出男女学员人数的差值
  select (select count(*) from student where stu_sex = "男") - (select count(*) from student where stu_sex = "女") as "男女人数的差值";