Bộ dữ liệu luyện tập

select c.city, count(*) as so_don
from orders o
join customers c on c.id = o.customer_id
where o.status <> 'cancelled'
group by c.city
order by so_don desc;

select plan, count(*) as so_nguoi_dang_dung
from subscriptions
where ended_on is null
group by plan
order by so_nguoi_dang_dung desc;

select type, sum(amount) as tong_tien
from transactions
group by type
order by tong_tien desc;

select d.name as phong_ban, avg(e.salary) as luong_trung_binh
from employees e
join departments d on d.id = e.department_id
group by d.name
order by luong_trung_binh desc;

select department, count(*) as so_lan_kham, sum(fee) as tong_phi
from visits
group by department
order by tong_phi desc;

select c.title, count(e.student_id) as so_hoc_vien
from edu_courses c
left join edu_enrollments e on e.course_id = c.id
group by c.title
order by so_hoc_vien desc;