题解 | 统计各个部门的工资记录数

解题思路：

将下面俩表左连接查出要的数据，在和第一个表左连接查出最终数据

select 
d1.dept_no as dept_no,
d1.dept_name as dept_name,
d2.sum as sum 
from departments d1 
left join (
    select d.dept_no as dept_no,count(s.salary) as sum 
    from dept_emp d
    left join salaries s
    on  d.emp_no = s.emp_no 
    group by d.dept_no) d2
on d1.dept_no = d2.dept_no 
group by d1.dept_no;