Postgres：如何获取枚举集中的下一项？

Question

考虑使用下面的代码。类型 enum_buysell 仅包含 2 个值：buy 和 sell。在某些情况下我需要得到相反的值，但是代码看起来很难看，恕我直言。有没有办法优化它？我想过根本不使用枚举，例如改为布尔值，但这个想法并不完美，因为它使数据本身不那么明显。

select
  datetime,
  case
    when account_id_active  = p_account_id and direction = 'buy'  then 'buy'::enum_buysell
    when account_id_active  = p_account_id and direction = 'sell' then 'sell'::enum_buysell
    when account_id_passive = p_account_id and direction = 'buy'  then 'sell'::enum_buysell
    when account_id_passive = p_account_id and direction = 'sell' then 'buy'::enum_buysell
  end as direction,
  price,
  volume
from
  deals
where
  account_id_active  = p_account_id or
  account_id_passive = p_account_id
order by
  datetime desc
limit
  10;

Answer 1

由于PostgreSQL中没有获取枚举的下一个值的函数，您应该自己定义它。

create function next_buysell (e enum_buysell)
returns enum_buysell
as $$
begin
  return (case when e='buy'::enum_buysell then 'sell'::enum_buysell
               else 'buy'::enum_buysell
          end);
end
$$ language plpgsql;

现在，您可以像这样使用它：

postgres=# select next_buysell('sell'::enum_buysell);
 next_buysell
--------------
 buy
(1 row)

postgres=# select next_buysell('buy'::enum_buysell);
 next_buysell
--------------
 sell
(1 row)

您的 CASE 语句变为：

case
  when account_id_active  = p_account_id then direction
  when account_id_passive = p_account_id then next_buysell(direction)
end as direction

Postgres：如何获取枚举集中的下一项？

Postgres: How to get next item in an enum set?

postgresql

enums