问题

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？

示例：

假如数据输入的是上表，则输出结果如下：

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。

准备

create database leecote626;


use leecote626;


create table seat (
id int(20),
student varchar(20));


insert into seat values
(1,'Abbot'),
(2,'Doris'),
(3,'Emerson'),
(4,'Green'),
(5,'Jeames');

解答

#方法一：
select
    if(id%2=0,
        id-1,
        if(id=(select count(distinct id) from seat),
            id,
            id+1))
    as id,student
from seat
order by id;


#方法二：
SELECT
    (CASE
        WHEN MOD(id, 2) != 0 AND counts != id THEN id + 1
        WHEN MOD(id, 2) != 0 AND counts = id THEN id
        ELSE id - 1
    END) AS id,
    student
FROM
    seat,
    (SELECT
        COUNT(*) AS counts
    FROM
        seat) AS seat_counts
ORDER BY id ASC;


#方法三：
SELECT
    s1.id, COALESCE(s2.student, s1.student) AS student
FROM
    seat s1
        LEFT JOIN
    seat s2 ON ((s1.id + 1) ^ 1) - 1 = s2.id
ORDER BY s1.id;