SQL in leetcode——medium篇

https://leetcode.com/problems/exchange-seats/

交换相邻学生的id，很容易想到，id为基数的：id+=1；id为偶数：id-=1.需要注意的是，如果学生人数为奇数，最后一名同学是不能+1的。

步骤

• 首先考虑统计学生个数
  select count(*) as counts from seat
• 利用case语句写赋值逻辑

case
    when mod(id, 2) != 0 and id != counts then id += 1
    when mod(id, 2) != 0 and id == counts then id
    when mod(id, 2) == 0 then id -= 1
end

注意理解，这部分是id。

• 整体逻辑串起来

select
 (case
    when mod(id, 2) != 0 and id != counts then id + 1
    when mod(id, 2) != 0 and id = counts then id
    when mod(id, 2) = 0 then id - 1
end) as id, student
from seat, (select count(*) as counts from seat) as seat_count #seat_count为table
order by id asc #顺序