LeetCode 626. Exchange Seats

最新推荐文章于 2020-10-09 09:18:23 发布

转载最新推荐文章于 2020-10-09 09:18:23 发布 · 367 阅读

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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 ?

# 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

# 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 ?

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

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