Question

给定一个User表和Friend表，对于用户表中的每个用户，我想编写SQL查询来查找其朋友的共同朋友，即，对于User表中的UserID，查找彼此为朋友的现有朋友。例如

curl -H 'Accept-Encoding: br,gzip,deflate Host: cnpj.info Pragma: no-cache Cache-Control: no-cache Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3 Upgrade-Insecure-Requests: 1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3 --user-agent 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36' -X GET "http://cnpj.info/0101000"

我希望能够获得：

user_table
userID
1
2
5

friend_table
user1    user2
1        3   
1        4
3        4
5        3
5        6

我尝试了类似的方法来获取用户ID和他们的朋友：（添加UNION只是为了消除由于双向关系而造成的重复）

user friend1 friend2 
1    3       4

如何找到彼此是朋友的现有朋友？

Answer 1

您可以只查看朋友，看看他们是否有您关心的普通朋友。假设关系是双向的：

with friends as (
      select f.user
      from friend_table ft
      where ft.user1 = @user  -- the original friend
     )
select *
from friend_table ft join 
     friends f1
     on ft.user1 = f1.user join
     friends f2
     on ft.user2 = f2.user
where ft.user1 < ft.user2   -- eliminate duplicates;

SQL找到共同的朋友

1 个答案: