Sql 查詢獲取共同好友

Question

我有兩個表，一個稱為用戶（id，名字，姓氏......）第二個稱為關注，其中包含有關哪個用戶正在關注其他用戶的數據

下表示例

userId   followerId
6           10
6           2
6           3
8           1
8           2
8           3

如何獲取兩個用戶之間的共同好友數

預期結果應該是

first_user second_user num_of_mutual_friends
    83          73               3

Answer 1

WITH firstUser AS (
    SELECT followerId
    FROM followings
    WHERE userId = 6 
)
, secondUser AS (
    SELECT followerId
    FROM followings
    WHERE userId = 8 
)

SELECT COUNT(followerId)
FROM firstUser
INNER JOIN secondUser ON firstUser.followerId = secondUser.followerId

Answer 2

有多種方法可以實現您想要的，但這應該可以解決問題：

select
  f.userId as first_user,
  s.userId as second_user,
  count(*) as num_of_mutual_friends
from
  followings f
  inner join followings s on s.followerId = f.followerId
                         and s.userId = 6
where
  f.userId = 8
group by
  f.userId,
  s.userId

您可以在此處查看一個工作示例。

Answer 3

從用戶加入以下兩個：

select u.*
from users
join followings a on a.followerId = u.id
  and a.userid = 6
join followings b on b.followerId = u.id
  and b.userid = 8

Answer 4

您也可以使用子查詢來執行此操作。 因此，如果您想獲取 id 為 8 的用戶和 id 為 6 的用戶之間的相互關注者數量，可以使用以下命令：

解決方案 1：

SELECT COUNT(followerId) AS mutual_followers_count FROM followings WHERE userId = 6 AND followerId IN (SELECT followerId FROM followings WHERE userId = 8)

解決方案 2：

SELECT COUNT(DISTINCT(followerId)) AS mutual_followers_count FROM followings WHERE followerId in (SELECT followerId FROM followings WHERE userId = 6) AND followerId in (SELECT followerId FROM followings WHERE userId = 8)