mysql-----多表查询

#重点  ：   外链接语法

select 字段列表  from 表1 inner | left | right join  表2  on 表1.字段=表2.字段；

1、交叉链接：不适用任何匹配条件。生成笛卡尔积

select * from employee,department;

2、内连接：只连接匹配的行

select * from employee,department where employee.dep_id=department.id;  #不用这种形式
select * from employee inner join department on employee.dep_id=department.id;

3、外链接之左连接：优先显示左表全部记录

select * from employee left join department on employee.dep_id=department.id;
4、外链接之右连接：优先显示右表全部记录
select * from employee right join department on employee.dep_id=department.id;

5、全外链接：显示左右两个表全部记录

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

union

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

#示例1：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,即找出公司所有部门中年龄大于25岁的员工

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

select  t1.name,t1.age,t2.name from employee as t1 left join department as t2 on t1.dep_id=t2.id and age>25;

#示例2：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;

#1：子查询是将一个查询语句嵌套在另一个查询语句中。
#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4：还可以包含比较运算符：= 、 !=、> 、<等

#查询employee表，但dep_id必须在department表中出现过
select * from employee 
                            where dep_id in 
                            (select id from department);   

#比较运算符：=、!=、>、>=、<、<=、<>
#查询平均年龄在25岁以上的部门名
select id,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‘）；

#查看不足1人的部门名
select name from department where id in (
select dep.id from department dep left join employee emp on dep.id = emp.dep_id
    group by dep.id
    having count(emp.id) < 1
);



select * from department where id not in (
select dep_id from employee group by dep_id having count(id) >= 1
);

#deparment表中存在dep_id=200时，True
select * from employee where exists (select id from department where id=200);
因为括号中为TRUE，返回的值为select * from employee语句的结果

select * from employee where exists (select id from department where id=204);
因为括号中为Flase,外层不查询，返回空集Empty set