LeetCode - 626. 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.

# Write your MySQL query statement below
SELECT
    s.id,
    s.student
FROM
    (
        SELECT
            id - 1 AS id,
            student
        FROM
            seat
        WHERE
            (id % 2 = 0)
        UNION
            SELECT
                (CASE WHEN (cnt%2=1) AND id=cnt THEN id ELSE id + 1 END) AS id,
                student
            FROM
                seat,
                (select count(*) as cnt from seat) as seatcnt
            WHERE
                (id % 2 = 1)
    ) s
GROUP BY
    s.id ASC