题解 | #统计各个部门的工资记录数#

1.salaries左连接dept_emp，建立新表deptNo_sum记录dept_no和sum
2.departments内连接deptNo_sum

from departments depts 
join   #[departments内连接deptNo_sum]
(select de.dept_no,count(*) as sum from salaries s 
left join dept_emp de on s.emp_no = de.emp_no
group by de.dept_no) deptNo_sum   #[salaries左连接dept_emp，新表deptNo_sum记录了dept_no和sum]
on depts.dept_no=deptNo_sum.dept_no
order by depts.dept_no;