Home/ Questions/Q 7056189
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-28T03:50:16+00:00

I have written this query which returns a users friends and the friends on

  • 0

I have written this query which returns a users friends and the friends on those friends. As there is a lot of sub queries going on here I thought there much be a more efficient way of writing it, but it’s a bit beyond me.

users table

++++++++++++++++++
+ user_id + name +
++++++++++++++++++
+ 1       + bill +
+ 2       + bob  +
+ 3       + sam  +
+ 4       + ben  +
++++++++++++++++++

user_friendships table

+++++++++++++++++++++++++++++++++++++
+ sender_user_id + receiver_user_id +
+++++++++++++++++++++++++++++++++++++
+ 1              + 2                +
+ 2              + 3                +
+ 4              + 2                +
+++++++++++++++++++++++++++++++++++++

The table is bi-directional so user 1 is a friend of user 2 and user 2 is a friend of user 1.

User 1 only has 1 friend user 2. User 2 has 2 friends, user 3 and user 4.

When the below query is run for user 1, user 2, 3 and 4 are returned.

query

SELECT * FROM users
WHERE ( user_id IN 

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id IN

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id = '1' 
UNION 
SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id = '1')

UNION

SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id IN

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id = '1' 
UNION 
SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id = '1')

)

OR user_id IN

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id = '1' 
UNION 
SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id = '1')
)
AND user_id != '1'

To clarify the end result of the friends and friends of friends query should be a single list of user_ids so that it can be joined with the user’s table to retrieve the name etc

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    This could be helpfull: It tries to traverse the tables as few times as possible.

    -- First generation: Friends
SELECT
  IF(firstgen.sender_user_id=<your-user-id>,firstgen.receiver_user_id,firstgen.sender_user_id) AS friend
FROM
  user_friendships AS firstgen
WHERE
  firstgen.receiver_user_id=<your-user-id>
  OR firstgen.sender_user_id=<your-user-id>

UNION

-- Second generation: Friends of friends
SELECT
  IF(secondgen.sender_user_id in(firstgen.sender_user_id,firstgen.receiver_user_id),secondgen.receiver_user_id,secondgen.sender_user_id) AS friend
FROM
  user_friendships AS firstgen
  INNER JOIN user_friendships AS secondgen ON
    (firstgen.sender_user_id=<your-user-id> AND (secondgen.sender_user_id=firstgen.receiver_user_id OR secondgen.receiver_user_id=firstgen.receiver_user_id))
  OR
    (firstgen.receiver_user_id=<your-user-id> AND (secondgen.sender_user_id=firstgen.sender_user_id OR secondgen.receiver_user_id=firstgen.sender_user_id))
WHERE
  firstgen.receiver_user_id=<your-user-id>
  OR firstgen.sender_user_id=<your-user-id>
    • 0