数据库的连接查询

首先创建表，并在表中插入数据

--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');

对上面的数据进行查询实验:

inner join

内关联，连接查询：多个表连接起来查询，查询结果多个表的交集

left join

左连接，以左面的为基准，左边有但是右边没有提供可匹配的，左边没可匹配项的显示null

right join

右链接，以右边的为准，右边有但是左边没有匹配的，右边没有可匹配项的显示null

自关联

自己关联自己，在一个表中，

并且数据结构都一样，查询时需要给表七别名，否则会出错。

-- 连接查询
-- 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;
--给数据表起别名
select s.name, c.name from students as s inner join classes as c on s.cls_id=c.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;

-- 自关联
create table areas(
aid int primary key auto_increment,
name varchar(20),
pid int
);
-- 查询出河北省所有市

--需要起别名
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');