【leetcode】626. Exchange Seats

题目：

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.

分析：

先写一条子查询语句，判断总数是奇数还是偶数

sql语句：

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