数据库——连接、多表查询

--创建学生表

create table students (
id int unsigned not null auto_increment primary key,
name varchar(20) default '',
age tinyint unsigned default 0,
high decimal(5,2),
gender enum('男', '女', '中性', '保密') default '保密',
cls_id int unsigned default 0,
is_delete bit default 0
);
 

--创建班级表

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

--往students表里插入数据

insert into students values
(0,'小明',18,180.00,2,1,0),
(0,'小月月',19,180.00,2,2,0),
(0,'彭于晏',28,185.00,1,1,0),
(0,'刘德华',58,175.00,1,2,0),
(0,'黄蓉',108,160.00,2,1,0),
(0,'凤姐',44,150.00,4,2,1),
(0,'王祖贤',52,170.00,2,1,1),
(0,'周杰伦儿',34,null,1,1,0),
(0,'程坤',44,181.00,1,2,0),
(0,'和珅',55,166.00,2,2,0),
(0,'刘亦菲',29,162.00,3,3,0),
(0,'金星',45,180.00,2,4,0),
(0,'静香',18,170.00,1,4,0),
(0,'郭静',22,167.00,2,5,0),
(0,'周杰',33,178.00,1,1,0),
(0,'钱小豪',56,178.00,1,1,0),
(0,'谢霆锋',38,175.00,1,1,0);

--向classes表里插入数据

insert into classes values (0, '云唯_01期'),(0, '云唯_02期');

连接查询（内关联）

-- inner join ... on
　　-- 两个表连接查询

select * from students inner join classes

　　-- 查询能够对应班级的学生以及班级信息

select * from students inner join classes on students.cls_id=classes.id;

　　-- 按照要求显示姓名，班级

select students.*, classes.name from students inner join classes on students.cls_id=classes.id;　

　　-- 给数据表起名字

select s.name, c.name from students as s inner join classes as c on s.cls_id=c.id;　　

　　-- 查询 有能够对应班级的学生以及班级信息，显示学生的所有信息，只显示班级名称

select students.*, classes.name from students inner join classes on students.cls_id=classes.id;　

　　-- 在以上查询中将班级姓名显示在第一列

select classes.name,students.* from students inner join classes on students.cls_id=classes.id;　　

　　-- 查询有能够对应班级的学生以及班级信息，按照班级进行排序

select classes.id, students.* from students inner join classes on students.cls_id=classes.id order by classes.id;　

　　-- 当是同一个班级的时候，按照学生的id从小到大

select classes.id, students.* from students inner join classes on students.cls_id=classes.id order by classes.id, students.id;

连接查询（左关联，右关联）

　　--左关联，以左边表为基准，条件对应不上的显示null

select * from students left join classes on students.cls_id=classes.id;　　

　　　　

　　--右关联，以后边为基准，条件对应不上的显示null

select * from classes left join students on students.cls_id=classes.id;

　　

自关联

create table areas(
aid int primary key auto_increment,
name varchar(20),
pid int
);

　　

use test;
--省份
insert into areas(aid,name,pid) values(1,'北京市',null); 
insert into areas(aid,name,pid) values(0,'天津市',null); 
insert into areas(aid,name,pid) values(0,'河北省',null); 
insert into areas(aid,name,pid) values(0,'山西省',null);

--地级市
insert into areas(aid,name,pid) values(0,'海淀区',1); 
insert into areas(aid,name,pid) values(0,'滨海区',2); 
insert into areas(aid,name,pid) values(0,'沧州市',3); 
insert into areas(aid,name,pid) values(0,'大同市',4);

insert into areas(aid,name,pid) values(0,'朝阳区',1); 
insert into areas(aid,name,pid) values(0,'武清区',2); 
insert into areas(aid,name,pid) values(0,'石家庄',3); 
insert into areas(aid,name,pid) values(0,'太原市',4);

--县级市
insert into areas(aid,name,pid) values(0,'西二旗',5); 
insert into areas(aid,name,pid) values(0,'大港',6); 
insert into areas(aid,name,pid) values(0,'任丘市',7); 
insert into areas(aid,name,pid) values(0,'清徐',8);

insert into areas(aid,name,pid) values(0,'中关村',5); 
insert into areas(aid,name,pid) values(0,'汉沽',6); 
insert into areas(aid,name,pid) values(0,'河间市',7); 
insert into areas(aid,name,pid) values(0,'阳曲',8);　

　　-- 查询出河北省所有市

select * from areas as province inner join areas as city on province.aid=city.pid having name='河北省';

select province.name,city.name from areas as province inner join areas as city on province.aid=city.pid having province.name='河北省';

-- 子查询

　　-- 标量子查询
　　-- 查询出北京市所有区的信息
select * from areas where pid=(select aid from areas where name='北京市');
select * from areas where pid in (select aid from areas where name='北京市');