详解三大范式以及TopN问题
MSQL三大范式
TopN问题
类似于查找分组中最大或者前几个信息，也就是这类的问题不能group by 来解决
-- order by
select *from worker order by age DESC;
select *from worker limit 0,3;
select *from worker limit 3;
-- from 获取数据 0
-- select 显示数据 3
-- order by 排序 2
-- limit 分页
-- 先排序后分页 3
-- from -> order by -> limit -> select
select *from worker order by age limit 3;
-- 求平均年龄
select FLOOR(avg(age)) as '平均年龄' from worker;
-- 求大于平均年龄的人
-- 求大于平均年龄的男生?
-- 错误写法：select *from worker age>avg(age);
select *from worker where age>(select FLOOR(avg(age)) as '平均年龄' from worker) and sex='1';
select *,avg(age) from worker; # 只能显示一个，如何将所有的信息后面都加上平均年龄呢？
-- 重新创建一个表：
-- id name 科目 学分 成绩
-- 1 zs 数学 2 90
-- 2 ls 语文 3 80
-- 1 zs 语文 2 60
-- 会出现问题
create table student(
id int(11) NOT NULL AUTO_INCREMENT,
name varchar(255) NOT NULL,
age int(11) DEFAULT null,
sex enum('0','1') DEFAULT NULL,
PRIMARY KEY(id)
);
insert into student(name,age,sex)VALUES("李四",19,"1");
insert into student(name,age,sex)VALUES("王五",20,"1");
insert into student(name,age,sex)VALUES("赵六",21,"1");
insert into student(name,age,sex)VALUES("钱琪",22,"1");
insert into student(name,age,sex)VALUES("杨幂",18,"0");
insert into student(name,age,sex)VALUES("丽丽",19,"0");
insert into student(name,age,sex)VALUES("莎莎",20,"0");
insert into student(name,age,sex)VALUES("慧慧",22,"0");
insert into student(name,age,sex)VALUES("翠翠",23,"0");
create table score(
id int(11) NOT NULL AUTO_INCREMENT,
subjectName varchar(255) NOT NULL,
score int(11) NOT NULL,
studentId int(11) NOT NULL,
PRIMARY KEY(id)
);
insert into score(subjectName,score,studentId)VALUES("语文",80,1001);
insert into score(subjectName,score,studentId)VALUES("数学",89,1001);
insert into score(subjectName,score,studentId)VALUES("英语",73,1001);
insert into score(subjectName,score,studentId)VALUES("语文",81,1002);
insert into score(subjectName,score,studentId)VALUES("数学",99,1002);
insert into score(subjectName,score,studentId)VALUES("英语",94,1002);
insert into score(subjectName,score,studentId)VALUES("语文",65,1003);
insert into score(subjectName,score,studentId)VALUES("数学",45,1003);
insert into score(subjectName,score,studentId)VALUES("英语",12,1003);
insert into score(subjectName,score,studentId)VALUES("语文",33,1004);
insert into score(subjectName,score,studentId)VALUES("数学",68,1004);
insert into score(subjectName,score,studentId)VALUES("英语",59,1004);
insert into score(subjectName,score,studentId)VALUES("语文",19,1005);
insert into score(subjectName,score,studentId)VALUES("数学",100,1005);
insert into score(subjectName,score,studentId)VALUES("英语",80,1005);
insert into score(subjectName,score,studentId)VALUES("语文",85,1006);
insert into score(subjectName,score,studentId)VALUES("数学",88,1006);
insert into score(subjectName,score,studentId)VALUES("英语",73,1006);
insert into score(subjectName,score,studentId)VALUES("语文",88,1007);
insert into score(subjectName,score,studentId)VALUES("数学",45,1007);
insert into score(subjectName,score,studentId)VALUES("英语",89,1007);
insert into score(subjectName,score,studentId)VALUES("语文",51,1008);
insert into score(subjectName,score,studentId)VALUES("数学",42,1008);
insert into score(subjectName,score,studentId)VALUES("英语",43,1008);
insert into score(subjectName,score,studentId)VALUES("语文",90,1009);
insert into score(subjectName,score,studentId)VALUES("数学",56,1009);
insert into score(subjectName,score,studentId)VALUES("英语",69,1009);
insert into score(subjectName,score,studentId)VALUES("语文",99,1010);
insert into score(subjectName,score,studentId)VALUES("数学",63,1010);
insert into score(subjectName,score,studentId)VALUES("英语",88,1010);
-- 求学生年龄最大值
select max(age) from student;
-- 求男女当中的最大值
select *from student;
select *,max(age)'最大值',sum(age) '总数' from student GROUP BY sex;
-- 1005 杨幂 18 0 23
-- 1001 李四 19 1 22
-- 结果为两条:
-- 分组： 求的是组的数，而不是组内容的数
-- 且为性别分类的第一个：
-- 分组之后默认顺序的第一条
select sex,max(age)'最大值',sum(age) '总数' from student GROUP BY sex;
-- 将sex和age组合在一起分组
select sex,max(age)'最大值',sum(age) '总数' from student GROUP BY sex,age;
-- 求所有的最大值
select max(age) from student;
-- 求age最大的学生
select * from student where age=(select max(age) from student);
-- 求每组最大的值
select sex,max(age) from student GROUP BY sex;
-- 求每组age最大的学生
-- 错误写法：
select max(age) from student where age=(select sex,max(age) from student GROUP BY sex);
-- 错误写法2：TopN问题（查询的结果不对）
select * from student where age in (select max(age) from student GROUP BY sex);
-- group BY 不适合每组最大的问题
-- Top1 每组最大的学生
select * from student s1 where 0=(select COUNT(*) from student s2 where s1.sex = s2.sex and s1.age<s2.age)
-- Top 每组前三个大的学生
select * from student s1 where 3>(select COUNT(*) from student s2 where s1.sex = s2.sex and s1.age<s2.age)
-- 解决group by 被限制的问题
set sql_mode ='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';
-- 分页
select *from worker LIMIT 0,5;
-- group by
select *from worker;
select * from worker group by sex;
