思路：以员工_奖金表为主表连接员工表和薪水表
select
e1.emp_no
,first_name
,last_name
,btype
,salary
,case when btype = 1 then 0.1salary
when btype = 2 then 0.2salary
else 0.3*salary end bonus
from emp_bonus e1
left join employees e2 on e1.emp_no = e2.emp_no
left join salaries s on e1.emp_no = s.emp_no
where s.to_date = '9999-01-01'