mysql - 在 SQL 中向任一方向计算“朋友”

Question

我有一个模型，人们可以互相“交友”（友谊）。我如何查询一个人既是朋友又是朋友的人的数量，而不计算两次双向友谊？

这是一个例子：

1 -> 2
2 -> 1
1 -> 3
4 -> 1

我希望将其注册为拥有 3 个朋友的 #1

友谊（id、person_id、friend_id）

Answer 1

Select
  count(distinct(f.user_id + f.friend_id))
From
  Friends f
Where
  f.user_id = 1 or f.friend_id = 1

不过，这样做可能会更有效：

Select
  Count(*)
From (
  Select friend_id From Friends f Where f.user_id = 1
  Union
  Select user_id From Friends f where f.friend_id = 1
) as a

为了让每个人的朋友数，假设还有一个 users 表：

Select
  u.user_id,
  count(distinct f.user_id + f.friend_id)
From
  Users u
    Left Outer Join
  Friends f
    On u.user_id = f.user_id Or u.user_id = f.friend_id

尽管使用 or 加入通常意味着查询速度较慢。另一种方法是：

Select
  u.user_id,
  count(distinct f.friend_id)
From
  Users u
    Left Outer Join (
      Select user_id, friend_id from Friends
      Union All
      Select friend_id, user_id from Friends
  ) f
    On u.user_id = f.user_id

您可以将 Union All 更改为 Union 并摆脱独特的，不确定哪个会更快。

Answer 2

select person_id, sum(ct) friend_count
from (select person_id, count(*) ct from friends
      group by person_id
      UNION ALL
      select f1.friend_id, count(*) ct
      from friends f1
      left join friends f2 on f1.person_id = f2.friend_id
      where f2.person_id is null
      group by f1.friend_id) u
group by person_id

Answer 3

Select distinct f1.id,count(f1.id) 
from friendships f1 
join friendships f2 on f1.person_id = f2.friend_id
Group by f1.id

Answer 4

select id, count(*)
from
  (select id, person_id as p1, friend_id as p2 from friendships
   union select id, person_id as p2, friend_id as p1 from friendships) fs
group by id