6.查找所有员工入职时候的薪水情况

法一：hire_data时间=from_data时间

select s.emp_no,s.salary
from salaries s inner join employees e on s.emp_no=e.emp_no 
and s.from_date=e.hire_date
order by s.emp_no desc

法二：找到工资表中最小的from_data

SELECT emp_no,salary 
FROM salaries 
GROUP BY emp_no 
HAVING min( from_date ) 
ORDER BY emp_no DESC

法三：找到工资表中最小的from_data

select a.emp_no,s.salary
from salaries s,
(select emp_no,min(from_date)as min_from_date
from salaries
group by emp_no)a
where a.emp_no=s.emp_no and a.min_from_date=s.from_date
order by a.emp_no desc