梧桐数据库与`mysql`及`oracle`关于交换服务器编号的`SQL`写法分析(推荐)

create table servers (
    server_id int primary key,
    position int
);

create table servers (
    server_id int primary key,
    position int
);

create table servers (
    server_id number primary key,
    position number
);

insert into servers values (1,1);
insert into servers values (2,2);
insert into servers values (3,3);
insert into servers values (4,4);
insert into servers values (5,5);
insert into servers values (6,6);
insert into servers values (7,7);

select 
    case when server_id % 2 = 0 
         then server_id - 1
         when server_id = (select count(distinct server_id) from servers) and server_id % 2 = 1 
         then server_id
         else server_id + 1
     end as server_id,
    position
from servers
order by server_id;

select 
	server_id,
	decode(mod(server_id,2), 1, lead(position, 1, position) over(order by server_id), lag(position, 1) over(order by server_id)) as position
from servers

server_id % 2 = 0: 当 server_id 是偶数时，将其减去 1。
server_id = (select count(distinct server_id) from servers) and server_id % 2 = 1: 当 server_id 是奇数且为最大值时，保持不变。
否则，将其加 1。

order by server_id: 按照 server_id 排序。

mod(server_id, 2): 计算 server_id 除以 2 的余数，用来判断 server_id 是否为奇数
lead(position, 1, position) over (order by server_id): 余数为奇数时，获取当前行后面一行的 position 值，如果不存在，则返回当前行的 position 值。
lag(position, 1) over (order by server_id): 余数为偶数时，获取当前行前面一行的 position 值，如果不存在，则返回当前行的 position 值。

over (order by server_id): 按照 server_id 排序。