Get real friends from friendlist - mysql

Question

I know there are plenty of results on this topic, but they didn't help me.
I have a friends table with user1 and user2.
A real friend is when user1 is friend with user2 and user2 is friends with user1.
A friend request is when user1 is friends with user2. It looks something like this:
user1 | user2
-------------
1 | 2
2 | 1
1 | 3
3 | 1
1 | 5
How could the query look to get the real friends of #1?
I tried this but it returned null:
SELECT user2 FROM friends WHERE user1 = 1 AND user2 = 1
Also how would the query look for the friend request?

Answer 1

SELECT a.user1 FROM friends AS a JOIN friends AS b
ON a.user2 = b.user1 AND a.user1 = b.user2
WHERE a.user2 = ?
Where ? denote the ID the the "original" user.

Answer 2

One way to get this is with a JOIN operation:
SELECT f.user2
FROM friends f
JOIN friends r
ON r.user1 = f.user2
AND r.user2 = f.user1
WHERE f.user1 = 1
Given that a "real friend" relationship is identified by the existence of two tuples, that is, a real friend relationship between 1 and n would be represented by two rows in the table: (1,n) and (n,1).
The predicates in the join condition limit the rows returned to those rows that have a matching "inverse" tuple.
NOTE: a JOIN operation usually performs better than an equivalent IN (subquery) or EXISTS (subquery) patterns, but that performance difference is negligible with small sets. It's with larger sets that the performance difference becomes noticeable.
An equivalent result can be returned (usually less efficiently) using an EXISTS predicate:
SELECT f.user2
FROM friends f
WHERE f.user1 = 1
AND EXISTS ( SELECT 1
FROM friends r
WHERE r.user1 = f.user2
AND r.user2 = f.user1
)
or an IN predicate:
SELECT f.user2
FROM friends f
WHERE f.user1 = 1
AND f.user2 IN ( SELECT r.user1
FROM friends r
WHERE r.user2 = f.user1
)
(If there's not a unique constraint on friends(user1,user2), then JOIN may return some duplicate rows which may not be returned by the other queries, but none of the queries guarantee that no duplicates are returned. If there's no unique constraint, and you don't want any duplicates returned, then you can either add a DISTINCT keyword after the SELECT at the beginning of any of those statements, -or- add a GROUP BY f.user2 at the end of any of those statements.
To make the result set more deterministic (i.e. return the same result each time the query is run), you could add an ORDER BY clause. (But it's not needed with the GROUP BY since MySQL implicitly does an ORDER BY on the GROUP BY expressions.)
FOLLOWUP
explain how I could bind this results with the name in the user table? thank you. And how do I get the "not real" friends?
To get the name from the user table, we just add a JOIN to the user table, assuming id is the primary key column, and the user1 and user2 columns are foreign keys to the user table...
SELECT f.user2
, u.name
FROM friends f
JOIN user u
ON u.id = f.user2
JOIN friends r
ON r.user1 = f.user2
AND r.user2 = f.user1
WHERE f.user1 = 1
A "not real" friends would be represented as a tuple (row in the table) (1,n) which does not have a corresponding inverse tuple (n,1). To find those rows we use an anti-join pattern, which is an OUTER join (return all rows from one side plus any matching rows), and then a predicate that excludes the rows where a match was found (checking for a NULL in a column that is guaranteed not to be null if there is a match is how we do that):
This will find all the (1,n) tuples where there isn't a matching (n,1):
SELECT f.user2
, u.name
FROM friends f
JOIN user u
ON u.id = f.user2
LEFT
JOIN friends r
ON r.user1 = f.user2
AND r.user2 = f.user1
WHERE r.user1 IS NULL
AND f.user1 = 1
We'd have to flip that around to get the other side, (n,1) rows which don't have a matching (1,n) row:
SELECT f.user1
FROM friends f
JOIN user u
ON u.id = f.user1
LEFT
JOIN friends r
ON r.user2 = f.user1
AND r.user1 = f.user2
WHERE r.user2 IS NULL
AND f.user2 = 1

Answer 3

SELECT user2 FROM friends
WHERE user1 = 1
AND user2 IN (SELECT user1 from friends where user2 = 1);