Postgres:如何获取枚举集中的下一个项目？

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,但代码看起来很难看,imho.有没有办法优化它？我认为根本不使用枚举,例如将其设为布尔值,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 中没有获取枚举下一个值的函数，因此您应该自己定义它。

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)

你的CASE陈述变成：

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