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;
