1、查询在职员工的现在薪水 
select emp_no,salary as now_salary
from salaries
where to_date='9999-01-01'
2、查询员工的入职薪水 
select emp_no,salary as salary
from salaries
where from_date in
(select hire_date
from employees)
3、1表和2表内连接，计算薪水涨幅并按升序排序 
select a.emp_no,b.now_salary-a.salary as growth
from
(select emp_no,salary as salary
from salaries
where from_date in
(select hire_date
from employees)) as a
join
(select emp_no,salary as now_salary
from salaries
where to_date='9999-01-01') as b
on a.emp_no=b.emp_no
order by growth;