580. Count Student Number in Departments

https://leetcode.com/articles/count-student-number-in-departments/

select d.dept_name dept_name,count(s.student_id) student_number from department d
left join student s on d.dept_id = s.dept_id
group by d.dept_id
order by student_number desc, d.dept_name;

--Caution: we should not use COUNT(*), because it takes account if expression is null.