Question

Here is the schema of my DB:

id | user1 | user2 | status  //status 1 means the user is a friend
 1     1       2      1
 2     3       1      1    
 3     1       4      1
 4     2       3      1
 5     4       2      1 

In the above example

1. userId 1 has 3 friends 2,3,4
2. userId 2 has 3 friends ie. 3,4,1
3. userId 3 has 2 friends ie: 1,2
4. userId 4 has 2 friends ie:1,2

I need to find mutual friends between 2 people.

Any Ideas,

Answer 1

I am not really sure if this is the best solution. But maybe something like this:

SELECT
    *
FROM
(
    SELECT
        CASE WHEN t2.user1=1 THEN t2.user2 ELSE t2.user1 END AS userId
    FROM
        t AS t2
    WHERE 1 IN (t2.user1,t2.user2)
    AND t2.[status]=1
) as tbl
WHERE EXISTS 
(
    SELECT
        NULL
    FROM
        t
    WHERE 2 IN (t.user1,t.user2)
    AND tbl.userId=(CASE WHEN t.user1=2 THEN t.user2 ELSE t.user1 END)
    AND t.[status]=1
)

This gets the mutual friends from user 1 and 2. Which is 3,4