day 35 多表查询

昨日补充

用 name=''比name is null快

单表操作

分组

group by

分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等

用法：

select 聚合函数，选取的字段 from 表名 group by 分组的字段；
group by:是分组的关键词
group by 必须和聚合函数一起出现
聚合函数：
	max : 求最大的
     min:求最小的
    count: 计数，数量
     sum：求和
    avg：平均数
where 条件语句和group by分组语句的先后顺序：
where>group by>having(************)

例子：

#以性别为例，进行分组，统计男生和女生的人数
select count(id),sex from employee group by sex;

#对部门进行分组，求出每个部门年龄最大的那个人；
select max(age),depart_id from employee group by depart_id;

# count和sum的区别：
select depart_id,count(age) from employee group by depart_id;
select depart_id,sum(age) from employee group by depart_id;

having

表示对group by 之后的数据，进行二次筛选

# 对部门进行分组，求出部门的年龄的平均数
select depart_id,avg(age) from employee group by depart_id;

# 找到部门平均年龄大于35的部门
select depart_id,avg(age) from employee group by depart_id having avg(age)>35;

使用的先后顺序：
where>group by>having(************)

升序 降序

order by

语法：

order by 字段名 asc #升序
order by 字段名 desc #降序
如果对多个字段进行排序，
比如：
	order by age desc,id asc;
    表示：先对age进行降序，如果age有相同的行，则对id进行升序
    
select * from employee order by age desc,id desc;

使用的先后顺序：
	where > group by > having > order by

limit

语法：

分页
limit offset,size
offset:行数据索引
size:取多少条数据
 
#从第一条开始取十条
select * from employee limit 0,10; 

# 从第十一条开始取十条
select * from employee limit 10,10;

使用的先后顺序：
where > group by > having > order by > limit 

多表操作

外键

使用原因：

​ a.减少占用的空间

​ b.只需要修改表一次，其余的表中数据就会进行相应的修改

一对多

使用方法：
	constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)；
    
例子：
create table department(
	id int auto_increment primary key,
    name varchar(32) not null default ''
)charset=utf8;

insert into department (name) values ('研发部');
insert into department (name) values ('运维部');
insert into department (name) values ('前台部');
insert into department (name) values ('小卖部');

create table userinfo(
	id int auto_increment primary key,
    name varchar(32) not null default '',
    depart_id int not null default 1,
    constraint fk_user_depart foreign key (depart_id) references department(id)
)charset=utf8;

insert into userinfo (name,depart_id) values ('zekai',1);
insert into userinfo (name,depart_id) values ('zekai',2);
insert into userinfo (name,depart_id) values ('zekai',3);
insert into userinfo (name,depart_id) values ('zekai',4);
insert into userinfo (name,depart_id) values ('zekai',1);
insert into userinfo (name,depart_id) values ('zekai',2);
insert into userinfo (name,depart_id) values ('zekai',1);#会报错

多对多

create table boy(
	id int auto_increment primary key,
	bname varchar(32) not null default ''
)charset=utf8;

insert into boy (bname) values ('zhangsan'),('lisi'),('wangeu');

create table girl(
	id int auto_increment primary key,
    gname varchar(32) not null default ''
)charset=utf8;

insert into girl (gname) values ('cuihua'),('gangdan'),('jianguo');

create table boy2girl(
	id int auto_increment primary key,
	bid int not null default 1,
	gid int not null default 1,
	constraint fk_boy2girl_boy foreign key (bid) references boy(id),
    constraint fk_boy2girl_girl foreign key (gid) references girl(id)
)charset=utf8;

insert into boy2girl (bid,gid) values (1,1),(1,2),(2,3),(3,3),(2,2);

select * from boy left join boy2girl on boy.id=boy2girl.bid left join girl on girl.id=boy2girl.gid;

一对一

例子：

user:
    id     name     age   salary
    1      zekai     18    5000
    2      zekai1    23    6000
    3      zekai2    19    3000
由于salary是比较敏感的字段，由此我们需要将此字段单独拆出来，变成一张独立的表
private:
    id     salary      uid(外键+unique)
    1       5000        1
    2       6000        2
    3       3000        3
 create table user(
	id int auto_increment primary key,
    name varchar(32) not null default ''
)charset=utf8;   

 insert into user (name) values ('zhangsan'),('lisi'),('wangeu');
    
create table priv(
	id int auto_increment primary key,
    salary varchar(32) not null default '',
    uid int not null default 1,
    
    constraint fk_priv_user foreign key (uid) references user(id),
    unique(uid)
)charset=utf8;

insert into priv (salary,uid) values (2000,1),(2800,2),(3000,3);

多表联查

left join .....on

select * from userinfo left join department on userinfo.depart_id=department.id;

right join ....on

select * from userinfo right join department on userinfo.depart_id=department.id;