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.

• solution 1：对照上表及其查询结果可以得知，当原id为奇数时，交换座位后的id变为id+1,当原id为偶数时，交换座位后的id变为id-1,另一个方面需要考虑的是，学生人数为奇数时，最后一个学生的id不变，故应当用子查询确定学生的人数，然后分情况讨论即可。

[sql] view plain copy

print ?

1. # Write your MySQL query statement below  
2. select (case  
3.        when id%2 != 0 and id != counts then id+1     
4.        when id%2 != 0 and id = counts then id  
5.        else id-1 end) as id,student  
6.        from seat, (select count(*) as counts from seat) as s order by id asc  

# Write your MySQL query statement below
select (case
       when id%2 != 0 and id != counts then id+1   
       when id%2 != 0 and id = counts then id
       else id-1 end) as id,student
       from seat, (select count(*) as counts from seat) as s order by id asc

[sql] view plain copy

print ?

1. SELECT  
2.     s.id,  
3.     s.student  
4. FROM  
5.     (SELECT id - 1 AS id, student FROM seat WHERE id%2 = 0  
6.      UNION  
7.      SELECT  
8.      (CASE WHEN (cnt%2=1) AND id=cnt THEN id ELSE id + 1 END) AS id, student  
9.      FROM seat, (select count(*) as cnt from seat) as seatcnt  
10.      WHERE id%2 = 1) s  
11. GROUP BY  
12.     s.id ASC  

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

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.

• solution 1：对照上表及其查询结果可以得知，当原id为奇数时，交换座位后的id变为id+1,当原id为偶数时，交换座位后的id变为id-1,另一个方面需要考虑的是，学生人数为奇数时，最后一个学生的id不变，故应当用子查询确定学生的人数，然后分情况讨论即可。

[sql] view plain copy

print ?

1. # Write your MySQL query statement below  
2. select (case  
3.        when id%2 != 0 and id != counts then id+1     
4.        when id%2 != 0 and id = counts then id  
5.        else id-1 end) as id,student  
6.        from seat, (select count(*) as counts from seat) as s order by id asc  

# Write your MySQL query statement below
select (case
       when id%2 != 0 and id != counts then id+1   
       when id%2 != 0 and id = counts then id
       else id-1 end) as id,student
       from seat, (select count(*) as counts from seat) as s order by id asc

[sql] view plain copy

print ?

1. SELECT  
2.     s.id,  
3.     s.student  
4. FROM  
5.     (SELECT id - 1 AS id, student FROM seat WHERE id%2 = 0  
6.      UNION  
7.      SELECT  
8.      (CASE WHEN (cnt%2=1) AND id=cnt THEN id ELSE id + 1 END) AS id, student  
9.      FROM seat, (select count(*) as cnt from seat) as seatcnt  
10.      WHERE id%2 = 1) s  
11. GROUP BY  
12.     s.id ASC  

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