1.求一个班级数学平均分。
(1). select sum(math) / count(math) as 数学平均分
from student;

(2). select avg(math) as 数学平均分
from student;

(3).select avg(name) as 小明平均分
from student;//0

2.求一个班级总分平均分。
(1).select (sum(chinese)+sum(math)+sum(english)) / count(*)
from student;

(2).select avg(chinese+math+english)
from student;

3.求班级语文最高分和最低分。
select max(name),min(name)
from student;

drop table if exists teacher;
create table teacher(
id int,
name varchar(20),
birthday date
);
insert into teacher(id,name,birthday) values(1,'jack','2011-1-1');
insert into teacher(id,name,birthday) values(2,'marry','2011-2-2');
insert into teacher(id,name,birthday) values(3,'sisi','2011-3-3');

select max(birthday),min(birthday)
from teacher;

4.对订单表中商品归类后，显示每一类商品的总价
select product as 类别名,sum(price) as 商品类别总价
from orders
group by product;

5.查询购买了几类商品，并且每类总价大于100的商品
select product as 类别名,sum(price) as 商品类别总价
from orders
group by product
having sum(price) > 100;

6.where v.s. having区别：

where主要用于行过滤器
having主要用于类别过滤器，通常有having就一定出现group by,但有group by的地方，不一定出现having。hving可以说是针对结果集在进行查询的。

drop table if exists teacher;
create table teacher(
id int primary key auto_increment,
name varchar(20) not null unique,
birthday date
);
insert into teacher(name,birthday) values(NULL,'2011-1-1');
insert into teacher(name,birthday) values('marry','2011-2-2');
insert into teacher(id,name,birthday) values(3,'sisi','2011-3-3');

select max(birthday),min(birthday)
from teacher;

7.删除主键，主键在表中只有一个，要么是一列，要么是多列
alter table teacher drop primary key;

8.一对一关系(方案一)：
drop table if exists card;
drop table if exists person;

create table person(
id int primary key auto_increment,
name varchar(20) not null
);

insert into person(name) values('jack');
insert into person(name) values('marry');

create table card(
id int primary key auto_increment,
location varchar(20) not null,
pid int,
constraint pid_FK foreign key(pid) references person(id)
);

insert into card(location,pid) values('BJ',1);
insert into card(location,pid) values('GZ',2);
insert into card(location,pid) values('CS',NULL);
insert into card(location,pid) values('NJ',3);//出错

//删除person表的某记录
delete from person where name = 'jack';

9.一对一关系(方案二)：
drop table if exists card;
drop table if exists person;

create table person(
id int primary key auto_increment,
name varchar(20) not null
);
insert into person(name) values('jack');
insert into person(name) values('marry');

create table card(
id int primary key auto_increment,
location varchar(20) not null,
constraint id_FK foreign key(id) references person(id)
);
insert into card(location) values('BJ');
insert into card(location) values('GZ');
insert into card(location) values('CS');//出错
insert into card(location) values(NULL);

10.一对多/多对一关系：
drop table if exists employee;
drop table if exists department;

create table department(
id int primary key auto_increment,
name varchar(20) not null
);
insert into department(name) values('软件部');
insert into department(name) values('销售部');

create table employee(
id int primary key auto_increment,
name varchar(20) not null,
did int,
constraint did_FK foreign key(did) references department(id)
);
insert into employee(name,did) values('jack',1);
insert into employee(name,did) values('marry',1);

11.问题？查询"软件部"的所有员工（组合式）
select d.name as 部门名,e.name as 员工名
from department as d,employee as e
where d.name = '软件部';

(1)select id from department where name='软件部';
(2)select name from employee where did = 1;
(总)嵌入式SQL

select name as 员工
from employee
where did = (
select id
from department
where name='软件部'
);

12.多对多关系：
drop table if exists middle;
drop table if exists student;
drop table if exists teacher;

create table if not exists student(
id int primary key auto_increment,
name varchar(20) not null
);
insert into student(name) values('jack');
insert into student(name) values('marry');

create table if not exists teacher(
id int primary key auto_increment,
name varchar(20) not null
);
insert into teacher(name) values('赵');
insert into teacher(name) values('蔡');

create table if not exists middle(
sid int,
tid int,
constraint sid_FK foreign key(sid) references student(id),
constraint tid_FK foreign key(tid) references teacher(id),
primary key(sid,tid)
);
insert into middle(sid,tid) values(1,1);
insert into middle(sid,tid) values(1,2);
insert into middle(sid,tid) values(2,1);
insert into middle(sid,tid) values(2,2);

13.问题？查询"赵"所教过的所有学员
select t.name as 老师, s.name as 学员
from teacher as t,student as s,middle as m
where t.name = '赵'and m.sid=s.id and m.tid=t.id;

14.模式：
select 列出需要显示的字段
from 列出所涉及到的所有表，建议写别名
where 业务条件 and 表关联条件

15.使用MySQL特有函数：

select datediff('2011-12-31',now());

16.截取字符串
select substring('mysql',1,2); //从1开始

17.保留小数点后2位(四舍五入)
select format(3.1415926535657989,3);

18.向下取整(截取)
select floor(3.14);
select floor(-3.14);
select floor(3.54);
select floor(-3.54);

19.取随机值
select format(rand(),2);

20.取1-6之间的随机整数值
select floor(rand()*6) + 1;

21.MySQL扩展知识：

(1)查询'a'-'z'对应的Unicode值
select ascii('a');//97
select ascii('z');//122

(2)产生97-122之间的随机整数
select floor(rand()*26)+97;

(3)产生97-122对应的字符
select char(floor(rand()*26)+97);

22.查MySQL文档，利用MySQL的函数：对密码'123456'进行。
select md5('123456');

drop table user;
create table user(
id int primary key auto_increment,
name varchar(20),
gender varchar(6),
salary float
);
insert into user(name,gender,salary) values('jack','male',4000);
insert into user(name,gender,salary) values('marry','female',5000);
insert into user(name,gender,salary) values('jim','male',6000);
insert into user(name,gender,salary) values('tom','male',7000);
insert into user(name,gender,salary) values('soso','female',NULL);
insert into user(name,gender,salary) values('haha','female',3500);
insert into user(name,gender,salary) values('hehe','female',4500);
select * from user;

23.MySQL特有流程控制函数：
1) if(value,第一值,第二值);
value为真，取第一值，否则取第二值

select if(salary>=5000,'高薪','起薪')
from user;

2) ifnull(value1,value2)
value1为NULL，用value2替代

select name as 员工,ifnull(salary,'无薪') as 薪水情况
from user;

3) case when [value] then [result1] else [result2] end;

select
case when salary>=5000 then '高薪'
else '起薪' end
from user;

4) case [express] when [value1] then [result1] when [value2] then [result2] else [result3] end;

select
case salary
when 7000 then '高薪'
when 6000 then '中薪'
when 5000 then '起薪'
else '低薪' end
from user;

25.查询相同性别的员工总人数>2的工资总和，并按工资总和降序排列
select count(*) as 员人数,gender as 性别,sum(salary) as 工资和
from user
group by gender
having count(*)>2
order by sum(salary) desc;

26.将性别为男的员工工资-1000，性别为女的员工工资+1000,在一条SQL上完成
select if(gender='female',salary+1000,salary-1000) as 工资 from user;

27.常用函数举例

select now();
select year(now());
select month(now());
select day(now());
select floor(datediff(now(),‘1999-01-01’)/365);//间隔年
select format(rand(),2);
select floor(rand()*5)+1;[1-5]随机值
select length(trim(' jack '));
select strcmp('a','w');

1 .关系的完整性

(1)实体(行)完整性：每条记录有一个唯一标识符，通常用无任何业务含义的字段表示
(2)参照完整性：一张（A）表的某个字段必须引用另一张（B）表的某个字段值，而且B表 的字段必须先存在。
(3)域(列)完整性：域即单元数据，域中的数值必须符合一定的规则，例如字段的值域、字 段的类型等的约束。

2 键的概念
(1)主键：只有唯一字段
(2)组合主键：由多个字段组合起来，形成唯一字段
(3)外键：针对多张表之间的关联

3 主键的特点
(1)主键不能重复
(2)主键不能为NULL
(3)auto_increment是MySQL特有的，默认从1开始，该ID值与表同生亡
(4)多人项目中，通常使用UUID来生成唯一的主键值，便于多个合并数据时依然保持实体完整性

4 唯一约束的特点
(1)非NULL值不能重复
(2)可以插入多个NULL值
(3)'NULL'空串和NULL是不同的概念

5 非空约束特点
(1)不能插入NULL值
(2)主键约束=非NULL约束+唯一约束

6 外健特点
(1)外键值必须来源于所引用别一个表主键值，或NULL

7 关联关系
(1)一对一(外健根业务有关)
(2)一对多或多对一(外键放置在多方)

(3)多对多(外健放置在关联表中，即将一个多对多拆分成二个一对多关系)

8.常用函数:
（1）.日期函数：

2.数学函数：

3.字符串函数