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');

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',
    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, 01);
insert into student values(02, '杜甫', '男', 20, 01);
insert into student values(03, '张飞', '男', 32, 02);
insert into student values(04, '韩信', '男', 26, 02);
insert into student values(05, '了龙', '男', 27, 02);
insert into student values(06, '大乔', '女', 17, 01);
insert into student values(07, '小乔', '女', 16, 01);
insert into student values(08, '小乔', '女', 16, 01);
insert into student values(09, '关哥', '男', 32, 02);
insert into student values(10, '刘备', '男', 36, null);
alter table student drop foreign key `student_ibfk_1`;


1: 查询出student表中年龄最大的学生
    select * from student where stu_age = (select max(stu_age) from student);

2: 查询出student表中年龄最小的学生
    select * from student where stu_age = (select min(stu_age) from student);

3: 查询出02号班中最大的年龄是多少
    select max(stu_age) from student where class_no = 2;

4: 查询出01号班中最小的年龄是多少
    select min(stu_age) from student  where class_no = 1; 

5: 查询出01号班中有多少个学生
    select count(*) from student where class_no = 1;

6: 查询出01号班中平均年龄是多少
    select avg(stu_age) from student where class_no = 1;

7：查询出没有班级的学生
    (失败)select * from student where class_no not in ( select class_no from class);(查询出结果为空?)
    (成功)select * from student where not exists ( select distinct(class_no) from class where student.class_no = class.class_no); // null值的特殊性,不能使用not in来查询,NULL值在与任意值比较时总是假的（FALSE），并且包含NULL的一个表达式总是产生一个NULL值

8: 查询出02号班级中年龄大于30岁的男学员
    select * from student where class_no = 2 and stu_sex = '男' and stu_age > 30;

9: 查询出所有的女学员姓名,并在名字后加上‘大美女’名字
    select concat(stu_name,'大美女') from student where stu_sex = '女';

10: 查询出1号班级的所有男学员 还有 没有班级的学员
     select * from student where (class_no = 1 || class_no is null) and stu_sex = '男';
     select * from student where class_no = 1 and stu_sex = '男' || not exists ( select class_no from class where student.class_no = class.class_no);

11: 查询出年龄在20-30岁的学员
    select * from student where stu_age between 20 and 30;

12: 查询出所有名字中第二个字符是‘乔’的学员的平均工资(没有工资列,改为平均年龄或者添加工资列)
    select avg(stu_age) from student where stu_name like('_乔%');

13: 查询出班中所有学生的姓名,其中不包括重复的
    select distinct(stu_name) from student where class_no is not null;

14: 查询姓名,性别两个字段, 不包括重复的记录
    select distinct stu_name,stu_age from student;
15: 查询出1号部门中所有的学员,根据年龄升序排序,年龄相同的则按照学号降序排序
    select * from student where class_no = 1 order by stu_age asc,stu_no desc;

16: 查询出最后一条数据
    (失败)select * from student limit (select count(stu_no)-1 from student),1;// 参数不能为表达式
    select * from student order by stu_no desc limit 1;

17: 查询出学号为6的后面的3条数据
    select * from student where stu_no > 6 limit 3;

18: 查询出学号为6的前面的3条数据
    select * from student where stu_no < 6 order by stu_no desc limit 3;

19: 删除最后一条记录,(在不知道最后一条记录id的情况)
    delete from student where stu_no in (select stu_no from  (select stu_no from student order by stu_no desc limit 1) as temtable);// MySQL不能指定更新的目标表在FROM子句,所以先将删除的数据放到临时表中再进行删除    delete from student order by stu_no desc limit 1;
    
20: 删除掉学号为6的后面的3条数据(同理(19))
    delete from student where stu_no in (select stu_no from (select stu_no from student where stu_no > 6 limit 3) as temtable);     delete from student where stu_no > 6 limit 3;