select employees.emp_no,employees.first_name,employees.last_name,b.btype,s.salary,
(case b.btype
when 1 then salary0.1
when 2 then salary0.2
else salary*0.3
end) as bonus
from employees
join emp_bonus as b on employees.emp_no=b.emp_no
join salaries as s on employees.emp_no=s.emp_no
where s.to_date='9999-01-01'

首先需要了解case-when-when-else-end的这个循环
然后这边有一个疑问在于 什么时候需要用join 什么时候可以直接跨表提取