MySQL中连接查询和子查询的问题

select 字段1, 字段2, ...
from 表1 {inner|lift|right} join 表2
on 连接条件;

select * from employee,department;

select emp.id,emp.name,emp.age,emp.dep_id,emp.gender,dep.id,dep.name
from employee as emp INNER JOIN department as dep
on emp.dep_id=dep.id;

select emp.id,emp.name,emp.age,emp.dep_id,emp.gender,dep.id,dep.name
from employee as emp left join department as dep
on emp.dep_id=dep.id;

select emp.id,emp.name,emp.age,emp.dep_id,emp.gender,dep.id,dep.name
from employee as emp right join department as dep
on emp.dep_id=dep.id;

select emp.id,emp.name,emp.age,emp.dep_id,emp.gender,dep.id,dep.name
from employee as emp left join department as dep
on emp.dep_id=dep.id
union
select emp.id,emp.name,emp.age,emp.dep_id,emp.gender,dep.id,dep.name
from employee as emp right join department as dep
on emp.dep_id=dep.id;

-- 查询平均年龄在20以上的部门名称
select name
from department
where id in (
select dep_id
from employee
group by dep_id
having avg(age) > 20);

-- 查询财务部员工姓名
select name 
from employee
where dep_id in (
select id 
from department 
where name='财务部');


-- 查询所有大于平均年龄的员工的年龄和姓名
select name,age 
from employee 
where age > (
select avg(age) from employee);