[걷기반] 10) 이젠 테이블이 2개입니다

다음과 같은 직원(employees) 테이블과 부서(departments) 테이블이 있습니다.

 

employees 테이블

id	department_id	name
1	101	르탄이
2	102	배캠이
3	103	구구이
4	101	이션이

 

departments 테이블

id	name
101	인사팀
102	마케팅팀
103	기술팀

38. 현재 존재하고 있는 총 부서의 수를 구하는 쿼리를 작성해주세요!

select count(*)
from departments;

 

39. 모든 직원과 그들이 속한 부서의 이름을 나열하는 쿼리를 작성해주세요!

select e.name, d.name
from employees e
join departments d on e.department_id=d.id;

 

40. '기술팀' 부서에 속한 직원들의 이름을 나열하는 쿼리를 작성해주세요!

select e.name
from employees e
join departments d on e.department_id=d.id
where d.name='기술팀';

 

41. 부서별로 직원 수를 계산하는 쿼리를 작성해주세요!

select d.name, count(*) as employment_count
from employees e
join departments d on e.department_id=d.id
group by d.id;

 

42. 직원이 없는 부서의 이름을 찾는 쿼리를 작성해주세요!

select d.name
from departments d
left join employees e on e.department_id=d.id
where e.id is null;

 

 

43. '마케팅팀' 부서에만 속한 직원들의 이름을 나열하는 쿼리를 작성해주세요!

select e.name
from departments d
join employees e on e.department_id=d.id
where d.name='마케팅팀';