知识点：分情况讨论 case when then 解释：情况1：Case when ... then... 情况2：When ... then ... 其他情况：Else ... 结束该字段的查询：End 新定义的查询

select e.emp_no,e.first_name,e.last_name,emp.btype,s.salary,
case when emp.btype=1  then   0.1*s.salary
when  emp.btype=2  then  0.2*s.salary
else  0.3*s.salary
end bonus 
from employees as e ,emp_bonus as emp ,salaries as s 
where e.emp_no=emp.emp_no
and   emp.emp_no=s.emp_no
and  s.to_date="9999-01-01"
order by emp_no asc