29 换座位
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题目
题目链接:换座位
改变相邻两同学的座位,如果学生人数是奇数,则最后一位同学不需要换座位。
create table seat (
id int,
student varchar(255)
);
insert into seat values
(1, 'Doris'),
(2, 'Abbot'),
(3, 'Green'),
(4, 'Emerson'),
(5, 'Jeames');
SQL:方法一
select
(
case
when mod(id, 2) != 0 and counts != id then id + 1
when mod(id, 2) != 0 and counts = id then id
else id - 1
end
) as id,
student
from seat,
(select count(*) as counts from seat) as count
order by id asc;
解析
mod(n, m)
对n
取余。- 先计算出一共有多少条数据,取名为
counts
- 用
mod
函数对id
取余- 如果等于
0
,说明是偶数,则id - 1
- 如果不等于
0
,说明是奇数,并且id
不等于counts
,则id + 1
- 如果不等于
0
,说明是奇数,并且id
等于counts
,则id
不变
- 如果等于
SQL:方法二
select s.id,
if(mod(s.id, 2) = 1, s.back, s.ahead) as student
from (
select
id, student,
lag(student, 1, student) over() as ahead,
lead(student, 1, student) over() as back
from seat
) as s;
解析
-
窗口函数
lag
从前往后取,跳过第一行,null
用seat.student
来填充id student ahead 1 Doris Doris 2 Abbot Doris 3 Green Abbot 4 Emerson Green 5 Jeames Emerson -
窗口函数
lead
从后往前取,跳过最后一行,null
用seat.student
来填充id student back 1 Doris Abbot 2 Abbot Green 3 Green Emerson 4 Emerson Jeames 5 Jeames Jeames -
再通过
mod
判断奇偶数,奇数取back
,偶数取ahead
。