考虑到有的员工可能没有奖金 所以后面要用inner join

select e.emp_no,first_name,last_name,btype,salary,
salary*(0.1*(if(btype>2,3,btype))) as bonus 
from employees e 
left join salaries s 
on e.emp_no =s.emp_no 
inner join emp_bonus eb 
on e.emp_no=eb.emp_no  where  s.to_date='9999-01-01'