数据库之多表查询

1.多表查询

提前创建两张表，部门表（department），员工表（employee）

create table department(
id int,
name varchar(20) 
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('nvshen','male',18,200),
('xiaomage','female',18,204)
;

1.1多表查询链接

交叉连接：它会生成笛卡尔积，这种不用，只做了解

select * from employee,department;

内连接：取出两张表共有部分（join---on---的使用）

select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

# 也可以这么写（推荐）
select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;

效果

外链接之左链接：优先显示左表全部记录

# 在内连接的基础上增加左边有，右边没有的结果
select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;

效果

外链接之右链接：优先显示右表全部记录

select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;

效果

全外链接：显示左右两个表全部记录

　　mysql通过使用union（使用union会自动去重，使用union all不会）关键字间接实现全外连接

select * from employee left join department on department.id=employee.dep_id
union
select * from employee right join department on department.id=employee.dep_id;

效果

1.2升级查询

　　以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25，即找出年龄大于25岁的员工,姓名以及员工所在的部门

select employee.name,employee.age,department.name from employee inner join department on employee.dep_id=department.id where employee.age>25;

　　以内连接的方式查询employee和department表，并且以age字段的升序方式显示

select employee.id,employee.name,employee.age,department.name from employee,department where employee.dep_id = department.id and age > 25 order by age asc;

带in关键字的子查询

#查询平均年龄在25岁以上的部门名
select department.name from department where id in(select dep_id from employee group by dep_id having avg(age)>25);

# 查看技术部员工姓名
select name from employee where dep_id in(select id from department where name ='技术'); 
或
select name from employee where dep_id = (select id from department where name='技术');

#查看空部门
select name from department where id not in(select dep_id from employee group by dep_id);

带比较运算符的子查询

# 查询大于所有人平均年龄的员工名与年龄
select name,age from employee where age>(select avg(age) from employee);

# 查询大于部门内平均年龄的员工名、年龄
select t1.name,t1.age from employee as t1 inner join 
    (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
    on t1.dep_id=t2.dep_id where t1.age>t2.avg_age;

带exists关键字的子查询

　　使用EXISTS关键字时，内层查询语句不返回查询的记录，而是返回True或False

 　　返回True时，外层查询语句将进行查询

　　返回False时，外层查询语句不进行查询

2.一张图告诉你left join，inner join的区别