Postgres 多对多：如何 select 记录精确计数的链接

Question

使用该架构（当然还有更多字段）我想 select 确切地聊天，其中 clients.id 例如 344,345，它们正好是两个。

但我找到了所有与其他客户聊天的地方，而不仅仅是他们两个。

_{（来源：zimagez.com）}

大图http://www.zimagez.com/zimage/107410861087108810861089-10871086-habtm0.php

SELECT 
  chat_rooms.id, 
  chat_rooms.token
FROM 
  chat_rooms, 
  clients, 
  chat_rooms_clients
WHERE 
  chat_rooms_clients.chat_room_id = chat_rooms.id AND
  chat_rooms_clients.client_id = clients.id AND
  clients.id IN (344,354) AND 
  chat_rooms.meeting_id IS NULL
group by chat_rooms.id
having count(clients.id) = 2
ORDER BY
  chat_rooms.created_at DESC;

我的应用程序是用 rails 编写的。

Answer 1

使用提供的测试数据：

set search_path = testschema, pg_catalog;
select cr.id, array_agg(crc.client_id order by crc.client_id), cr.token
from
    chat_rooms_clients crc
    inner join
    chat_rooms cr on cr.id = crc.chat_room_id
where cr.meeting_id is null
group by 1
having array_agg(crc.client_id order by crc.client_id) = array[2467,2471]::int[]
order by cr.created_at desc;
 id  |  array_agg  |                token
-----+-------------+--------------------------------------
 234 | {2467,2471} | bd36db10-3a89-40fe-a3e3-524b4278b718

请注意，对于此查询，没有必要加入 clients table。我认为在 chat_rooms_clients table:

中创建一个主键是明智的

create table chat_rooms_clients (
    chat_room_id integer not null,
    client_id integer not null,
    primary key (chat_room_id, client_id)
);

否则为了保险起见，需要在上面查询的array_agg中加上distinct：

having array_agg(distinct crc.client_id order by crc.client_id) = ...

如果聊天室创建者必须是聊天参与者之一，则对其进行过滤以提高性能：

where cr.meeting_id is null and cr.creator_id in (2467,2471)

Postgres 多对多：如何 select 记录精确计数的链接

Postgres many to many: how to select record with exactly counted links

postgresql

many-to-many

aggregate

has-and-belongs-to-many