题解 | #删除emp_no重复的记录，只保留最小的id对应的记录。#

连表查询

select d.dept_no,d.dept_name,count(s.salary) from departments d 
left join 
(select ss.salary, de.dept_no from dept_emp de join salaries ss on de.emp_no=ss.emp_no ) s 
on
d.dept_no=s.dept_no 
group by d.dept_no 
order by d.dept_no