直接三表联合，然后判断btype再进行bonus操作

select 
    *,
    ( case 
	 	   when btype = 1 then salary * 0.1 
           when btype = 2 then salary * 0.2 
	 	   else 			   salary * 0.3
	  end
    ) as bonus 
from(
    select 
        eb.emp_no,first_name,last_name,btype,salary
    from 
        salaries       as sa 
        join emp_bonus as eb on eb.emp_no = sa.emp_no
        join employees as em on em.emp_no = eb.emp_no
    where 
        to_date = '9999-01-01'
    ) as t1