mysql求差问题

Question

问题如果要查询stime最大和时间最小两条数据中，value字段的差，测试表结构附上，求大牛们指点
现有A表，建表：
CREATE TABLE a (
id int(11) DEFAULT NULL,
value varchar(10) DEFAULT NULL,
stime datetime DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

添加数据：
INSERT INTO a VALUES ('1', '2', '2018-05-28 14:39:06');
INSERT INTO a VALUES ('2', '8', '2018-05-28 17:39:32');
INSERT INTO a VALUES ('3', '18', '2018-05-28 20:40:12');
INSERT INTO a VALUES ('4', '25', '2018-05-29 14:40:31');

Answer 1

select

(select value from a where stime = (select max(stime) from a))
- 
(select value from a where stime = (select min(stime) from a));