select e.emp_no,e.first_name,e.last_name,eb.btype,s.salary,
case when eb.btype=1 then s.salary*0.1
     when eb.btype=2 then s.salary*0.2
     else s.salary*0.3
end bonus
from employees e
  join emp_bonus eb on e.emp_no=eb.emp_no
  join salaries s on e.emp_no=s.emp_no
where to_date='9999-01-01'

思路：利用case when 语句