Finding mutual friend in one way relationship table - mysql

Question

want mysql query for finding mutual friend between two friend but
I am maintain the friendship of user in one way relationship for ex.
first is users table
id name
1 abc
2 xyz
3 pqr
Now second table is friend
id user_id friend_id
1 1 2
2 1 3
3 2 3
Now here i can say that abc(id=1) is friend of xyz(id=2) now similar way the xyz is friend of abc but now i want to find mutual friend between abc(id=1) and xyz(id=2) that is pqr so I want mysql query for that.

Answer 1

REVISED
This query will consider the "one way" relationship of a row in the friend table to be a "two way" relationship. That is, it will consider a friend relationship: ('abc','xyz') to be equivalent to the inverse relationship: ('xyz','abc'). (NOTE: we don't have any guarantee that both rows won't appear in the table, so we need to be careful about that. The UNION operator conveniently eliminates duplicates for us.)
This query should satisfy the specification:
SELECT mf.id
, mf.name
FROM (
SELECT fr.user_id AS user_id
, fr.friend_id AS friend_id
FROM friend fr
JOIN users fru
ON fru.id = fr.user_id
WHERE fru.name IN ('abc','xyz')
UNION
SELECT fl.friend_id AS user_id
, fl.user_id AS friend_id
FROM friend fl
JOIN users flf
ON flf.id = fl.friend_id
WHERE flf.user IN ('abc','xyz')
) f
JOIN users mf
ON mf.id = f.friend_id
GROUP BY mf.id, mf.name
HAVING COUNT(1) = 2
ORDER BY mf.id, mf.name
SQL Fiddle here http://sqlfiddle.com/#!2/b23a5/2
A more detailed explanation of how we arrive at this is given below. The original queries below assumed that a row in the friend table represented a "one way" relationship, in that "'abc' ff 'xyz'" did not imply "'xyz' ff 'abc'". But additional comments from the OP hinted that this was not the case.
If there is a unique constraint on friend(user_id,friend_id), then one way to get the result would be to get all of the friends of each user, and get a count of rows for that friend. If the count is 2, then we know a particular friend_id appears for both user 'abc' and for 'xyz'
SELECT mf.id
, mf.name
FROM friend f
JOIN users uu
ON uu.id = f.user_id
JOIN users mf
ON mf.id = f.friend_id
WHERE uu.name IN ('abc','xyz')
GROUP BY mf.id, mf.name
HAVING COUNT(1) = 2
ORDER BY mf.id, mf.name
(This approach can also be extended to find a mutual friend of three or more users, by including more users in the IN list, and changing the value we compare the COUNT(1) to.
This isn't the only query that will return the specified resultset; there are other ways to get it as well.
Another way to get an equivalent result:
SELECT u.id
, u.name
FROM ( SELECT f1.friend_id
FROM friend f1
JOIN users u1
ON u1.id = f1.user_id
WHERE u1.name = 'abc'
) t1
JOIN ( SELECT f2.friend_id
FROM friend f2
JOIN users u2
ON u2.id = f2.user_id
WHERE u2.name = 'xyz'
) t2
ON t2.friend_id = t1.friend_id
JOIN users u
ON u.id = t1.friend_id
ORDER BY u.id, u.name
NOTES
These queries do not check whether user 'abc' is a friend of 'xyz' (the two user names specified in the WHERE clause). It is only finding the common friend of both 'abc' and 'xyz'.
FOLLOWUP
The queries above satisfy the specified requirements, and all the examples and test cases provided in the question.
Now it sounds as if you want a row in that relationship table to be considered a "two way" relationship rather than just a "one way" relationship. It sounds like you want to want to consider the friend relationship ('abc','xyz') equivalent to ('xyz','abc').
To get that, then all that needs to be done is to have the query create the inverse rows,, and that makes it easier to query. We just need to be careful that if both those rows ('abc','xyz') and ('xyz','abc') already exist, that we don't create duplicates of them when we invert them.
To create the inverse rows, we can use a query like this. (It's simpler to look at this when we don't have the JOIN to the users table, and we use just the id value:
SELECT fr.user_id
, fr.friend_id
FROM friend fr
WHERE fr.user_id IN (1,2)
UNION
SELECT fl.friend_id AS user_id
, fl.user_id AS friend_id
FROM friend fl
WHERE fl.friend_id IN (1,2)
It's simpler if we don't include the predicates on the user_id and friend_id table, but that could be a very large (and expensive) rowset to materialize.

Answer 2

try this:
given that you want to get the mutual friends of friends 1 & 2
select friend_id into #tbl1 from users where user_id = 1
select friend_id into #tbl2 from users where friend_id = 2
select id, name from users where id in(select friend_id from #tbl1 f1, #tbl2 f2 where f1.friend_id=f2.friend_id)