CREATE TABLE `t_demo` (
 `id` int(32) NOT NULL,
 `name` varchar(255) DEFAULT NULL,
 `age` int(2) DEFAULT NULL,
 `num` int(3) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

SELECT
  *,
CASE
  WHEN t.num >= 85 THEN
    '優秀' 
  WHEN t.num  90 AND t.num>= 60 THEN
    '一般'
  ELSE
    '不及格'
  END AS level
FROM
  t_demo t;