Exchange_Seats.sql

/*
Exchange Seats
https://leetcode.com/problems/exchange-seats/

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?

+---------+---------+
|    id   | student |
+---------+---------+
|    1    | Abbot   |
|    2    | Doris   |
|    3    | Emerson |
|    4    | Green   |
|    5    | Jeames  |
+---------+---------+
For the sample input, the output is:
 

+---------+---------+
|    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.
*/

WITH paresAImpares AS (
    SELECT (id - 1) AS id, student
    FROM seat
    WHERE MOD(id, 2) = 0
), imparesAPares AS (
    SELECT (id + 1) as id, student
    FROM seat
    WHERE MOD(id, 2) = 1
)

SELECT id, coalesce(paresAImpares.student, imparesAPares.student, seat.student) AS student
FROM seat LEFT JOIN paresAImpares USING(id) LEFT JOIN imparesAPares using(id)