先选出每个部门中最高的薪资

select de.dept_no,max(s.salary) as maxSalary 
from dept_emp de join salaries s 
on de.emp_no=s.emp_no 
group by de.dept_no

将员工信息、部门信息以及工资信息合成一张表

select de.emp_no,de.dept_no,s.salary 
from dept_emp de join salaries s 
on de.emp_no=s.emp_no

按照最高薪资和员工编号链接上述两张表

select a.dept_no,hh.emp_no,a.maxSalary 
from (select de.emp_no,de.dept_no,s.salary from dept_emp de join salaries s on de.emp_no=s.emp_no) hh 
join 
(select de.dept_no,max(s.salary) as maxSalary 
from dept_emp de join salaries s on de.emp_no=s.emp_no 
group by de.dept_no) a 
on hh.salary=a.maxSalary and hh.dept_no=a.dept_no;