626. Exchange Seats.md

626. Exchange Seats

Table: Seat

Column Name	Type
id	int
student	varchar

id is the primary key (unique value) column for this table. Each row of this table indicates the name and the ID of a student. id is a continuous increment.

Write a solution to swap the seat id of every two consecutive students. If the number of students is odd, the id of the last student is not swapped.

Return the result table ordered by id in ascending order.

The result format is in the following example.

Example 1:

Input: Seat table:

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

Output:

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

Explanation: Note that if the number of students is odd, there is no need to change the last one's seat.

SELECT (
    CASE
        -- 5%2=1 5=5 id=5 
        WHEN MOD(id,2) = 1 AND id = counts THEN id
        -- 3%2=1 3!=5 id=4 
        WHEN MOD(id,2) = 1 AND id != counts THEN id+1
        -- swapping 
        ELSE id-1
    END 
) AS id, student
FROM Seat, (select count(*) as counts from seat) as t
ORDER BY id;