如何连接表（通过关系表）？

Question

考虑以下表格：

人：

id: integer
name: string

朋友：

id1: integer
id2: integer

我想找到所有朋友，所以我使用这样的东西：

People.joins("JOIN friends ON friend.id1 = people.id JOIN friends ON friend.id2 = people.id")

哪个生成此SQL：

SELECT "people".* FROM "people" 
JOIN friends ON friends.id1 = people.id 
JOIN people ON friends.id2 = people.id

以下错误：

(pry) output error: #<ActiveRecord::StatementInvalid:
PG::DuplicateAlias: ERROR:  table name "people" specified more than once

多次指定people有什么问题？

Answer 1

给他们不同的别名：

SELECT p1.* FROM people p1
JOIN friends ON friends.id1 = p1.id 
JOIN people p2 ON friends.id2 = p2.id