select eb.emp_no,
first_name,
last_name,
btype,
salary,
round
(
    case btype when 1 then salary*0.1 when 2 then salary*0.2 else salary*0.3 end,1) bonus

from emp_bonus eb 
left join employees e on eb.emp_no = e.emp_no
left join salaries s on eb.emp_no = s.emp_no
where to_date = '9999-01-01'
order by emp_no

用条件判断函数case when 以及 数学函数里的round