题解 | #统计各个部门的工资记录数#

select d.dept_no, d.dept_name, 
    sum(if(s.salary is not null,1,0)) as sum
from departments as d
inner join dept_emp as de 
on d.dept_no = de.dept_no
inner join salaries as s 
on de.emp_no = s.emp_no
group by d.dept_no
order by d.dept_no