leetcode-626-Exchange-Seats

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描述

Mary is a teacher in a middle school and she has a table seat storing students’ names and their corresponding seat ids.

The column id is continuous increment.

Mary wants to change seats for the adjacent students.

Can you write a SQL query to output the result for Mary?

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+---------+---------+
|    id   | student |
+---------+---------+
|    1    | Abbot   |
|    2    | Doris   |
|    3    | Emerson |
|    4    | Green   |
|    5    | Jeames  |
+---------+---------+

For the sample input, the output is:

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+---------+---------+
|    id   | student |
+---------+---------+
|    1    | Doris   |
|    2    | Abbot   |
|    3    | Green   |
|    4    | Emerson |
|    5    | Jeames  |
+---------+---------+

Note:
If the number of students is odd, there is no need to change the last one’s seat.

分析

将表中的奇数行与偶数行交换位置，如果表的总行数是奇数，最后一行不变。

SQL 的 IF 函数类似于 A? B: C。

解决方案1(MySQL)

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# Write your MySQL query statement below
select if(id < (select count(*) from seat), if(id mod 2 = 0, id-1, id+1), if(id mod 2 = 0, id-1, id)) as id, student 
                                                                             from seat order by id asc;

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