【MySQL】基础知识-case when函数

case when 用法：

1. case when expression1 then XXX when expression2 then XXX end；
   例：select case when grade <60 then “不及格”
   when grade >=60 then “及格”
   end AS STATUS
   from class;

2. case column when A1 then XXX when A2 then XXX,eles XXX end;
   例：select case grade when 60 then “刚好及格”
   when 0 then “抓紧玩儿去”
   else “都太平庸”
   end AS FAIL
   from class;

3. 也可以将case when 用在Where条件中(例子可能不大合适，只是说明用法)
   例：#统计出属于low的罚款编号。重点看这里的解决方法 

select paymentno,amount from penalties
where case 
when amount>0 and amount<=40 then ‘low’ 
when amount>40 and amount<=80 then ‘moderate’ 
when amount>80 then ‘high’ 
else ‘incorrect’ end =’low’;