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-- @Title: 换座位 (Exchange Seats)
-- @Author: 15816537946@163.com
-- @Date: 2021-03-03 22:55:19
-- @Runtime: 199 ms
-- @Memory: 0 B
# Write your MySQL query statement below


## 处理偶数 id，让 id 减 1
## 例如 2,4,6,... 变成 1,3,5,...
SELECT
    s1.id - 1 AS id,
    s1.student
FROM
    seat s1
WHERE
    s1.id MOD 2 = 0 UNION
## 处理奇数 id，让 id 加 1。但是如果最大的 id 为奇数，则不做处理
## 例如 1,3,5,... 变成 2,4,6,...
SELECT
    s2.id + 1 AS id,
    s2.student
FROM
    seat s2
WHERE
    s2.id MOD 2 = 1
    AND s2.id != ( SELECT max( s3.id ) FROM seat s3 ) UNION
## 如果最大的 id 为奇数，单独取出这个数
SELECT
    s4.id AS id,
    s4.student
FROM
    seat s4
WHERE
    s4.id MOD 2 = 1
    AND s4.id = ( SELECT max( s5.id ) FROM seat s5 )
ORDER BY
    id;