将三张表连接起来，通过where to_date='9999-01-01'过滤出当前的薪水，然后根据btype判断奖金多少，就行了

case 
when b.btype=1 then round(c.salary*0.1,1)
when b.btype=2 then round(c.salary*0.2,1)
else round(c.salary*0.3,1)
end as bonus
from employees a
inner join emp_bonus b 
on a.emp_no=b.emp_no
inner join salaries c
on b.emp_no=c.emp_no 
where c.to_date='9999-01-01'
order by emp_no