626. 换座位
地址:https://leetcode-cn.com/problems/exchange-seats/
## 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? SQL架构 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。 `解题思路` 1 使用case then mod(id, 2) = 1 确定是否为奇数 `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; `