case when then end

case when 对字符串的处理，可以使用下边的第二种更方便

select *, (case  when name='zhangsan' then '张三' end) as '姓名' from testcase;
select *, (case  name when 'zhangsan' then '张三' end) as '姓名' from testcase;

但是对于数字类型判断大小就只能使用上边那种了

select *, (case when age>12 then '青少年' else '儿童' end) as '年龄段' from testcase;