项目8: 换座位(难度:中等)
小美是一所中学的信息科技老师,她有一张 seat 座位表,平时用来储存学生名字和与他们相对应的座位 id。
其中纵列的 id 是连续递增的,小美想改变相邻俩学生的座位,你能不能帮她写一个 SQL query 来输出小美想要的结果呢?
请创建如下所示seat表:
示例:
+---------+---------+
| id | student |
+---------+---------+
| 1 | Abbot |
| 2 | Doris |
| 3 | Emerson |
| 4 | Green |
| 5 | Jeames |
+---------+---------+
假如数据输入的是上表,则输出结果如下:
+---------+---------+
| id | student |
+---------+---------+
| 1 | Doris |
| 2 | Abbot |
| 3 | Green |
| 4 | Emerson |
| 5 | Jeames |
+---------+---------+
注意:如果学生人数是奇数,则不需要改变最后一个同学的座位。
-- 创建seat表 CREATE TABLE seat ( id int(11) NOT NULL, student varchar(20) NOT NULL, PRIMARY KEY (`id`)); -- 插入数据 INSERT INTO seat VALUES ('1', 'Abbot'); INSERT INTO seat VALUES ('2', 'Doris'); INSERT INTO seat VALUES ('3', 'Emerson'); INSERT INTO seat VALUES ('4', 'Green'); INSERT INTO seat VALUES ('5', 'Jeames');
查询SQL代码:
-- id为偶数时,将id-1 SELECT id-1 as id,student FROM seat WHERE id % 2=0 UNION -- id为奇数且最后一个数不是奇数时,将id+1 SELECT id+1 as id,student FROM seat WHERE id % 2=1 AND id < (SELECT COUNT(*) from seat) UNION -- id为奇数且最后一个数是奇数时,id不变 SELECT * FROM seat WHERE id % 2=1 AND id =(SELECT COUNT(*) from seat) ORDER BY id;