PostgreSQL函数迭代/作用于具有状态的许多行 [英] PostgreSQL function to iterate through/act on many rows with state

问题描述

我有一个数据库，其中的列看起来像这样：

I have a database with columns looking like:

session | order | atype | amt
--------+-------+-------+-----
1       |  0    | ADD   | 10
1       |  1    | ADD   | 20
1       |  2    | SET   | 35
1       |  3    | ADD   | 10
2       |  0    | SET   | 30
2       |  1    | ADD   | 20
2       |  2    | SET   | 55

它表示正在执行的操作。每个会话从0开始。ADD添加一个量，而SET设置一个量。我想要一个函数返回会话的结束值，例如

It represents actions happening. Each session starts at 0. ADD adds an amount, while SET sets it. I want a function to return the end value of a session, e.g.

SELECT session_val(1); --returns 45
SELECT session_val(2); --returns 55

是否可以编写这样的函数/查询？我不知道如何使用SQL做任何类似迭代的事情，或者根本不可能。

Is it possible to write such a function/query? I don't know how to do any iteration-like things with SQL, or if it's possible at all.

推荐答案

select sum(amt) as session_val
from (
  select segment,
         max(segment) over() as max_segment,
         amt
  from (
    select sum(case when atype = 'SET' then 1 else 0 end)
               over(order by "order") as segment,
           amt
    from command
    where session = 2
  ) x
) x
where segment = max_segment

虽然在PL / pgsql中非常简单：

It is quite simple in PL/pgsql though:

create function session_val(session int) returns int stable strict
language plpgsql as $$
declare
  value int := 0;
  rec command%rowtype;
begin
  for rec in select * from command where command.session = session_val.session loop
    if rec.atype = 'SET' then
      value := rec.amt;
    elsif rec.atype = 'ADD' then
      value := value + rec.amt;
    end if;
  end loop;
  return value;
end $$;

所以，请选择。

这篇关于PostgreSQL函数迭代/作用于具有状态的许多行的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！