一、多表查询

多表联合查询
# 方案1：链表
把多张物理表合并成一张虚拟表，再进行后续查询

#======>内链接：保留两张表有对应关系的记录
select * from emp,dep where emp.dep_id=dep.id;
select dep.name,emp.name from emp inner join dep on emp.dep_id=dep.id
    where dep.name = "技术";

#======>左链接：在内链接的基础上保留左表的记录
select * from emp left join dep on emp.dep_id=dep.id;

#======>右链接：在内链接的基础上保留右表的记录
select * from emp right join dep on emp.dep_id=dep.id;

#======>全外链接：在内链接的基础上保留左右表的记录
full join

select * from emp left join dep on emp.dep_id=dep.id
union
select * from emp right join dep on emp.dep_id=dep.id;


示例1：查询所有部门名及对应的员工个数
select dep.name,count(emp.id) from emp right join dep on emp.dep_id = dep.id
group by dep.name
;

select dep.name,count(emp.id) from emp right join dep on emp.dep_id = dep.id
group by dep.name
having count(emp.id) < 2
;

#示例2：即找出年龄大于25岁的员工以及员工所在的部门
select emp.name,dep.name from emp inner join dep on emp.dep_id = dep.id where age > 25;


#示例3：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


# 把多张表链接到一起：
select * from
(select emp.*,dep.name as dep_name  from emp inner join dep on emp.dep_id = dep.id) as t1
inner join
dep
on t1.dep_id = dep.id
;

select * from emp
inner join dep
on emp.dep_id = dep.id

inner join dep as t1
on t1.id = dep.id;


# 查询部门内最新入职的员工
select * from employee
inner join
(select depart_id,max(hire_date) as maxd from employee group by depart_id) as t1
on employee.depart_id = t1.depart_id
where employee.hire_date = t1.maxd
;


# 方案2：子查询
从一张表中查询出结果，用该结果作为查
询下一张表的过滤条件
select * from employee
where hire_date =  (select max(hire_date) from employee);


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

#查看技术部员工姓名
select * from emp where dep_id in
(select id from dep where name="技术");

#查看不足1人的部门名(子查询得到的是有人的部门id)
select * from dep where id not in (select distinct dep_id from emp);

select * from dep where exists (select * from emp where id>3);