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\');