Question

I have a MySQL table like this:

| USER | FRIEND |
-----------------
|    1 |      2 |
|    2 |      1 |
|    1 |     16 |
|   16 |      1 |
|    1 |      4 |

I need to write a MySQL query to list all (accepted) friends of user 1, in this case, it should return 2 and 16, but not 4, as that friendship isn’t accepted.

Is there any 1-query way to do this?

Answer 1

I need to write a MySQL query to list all (accepted) friends of user 1

Assuming that an accepted fried is a friend that has a two entries in the table, one from the user to the friend and another from the friend to the user(as you explained with the sample data in your question), then you can do this:

SELECT ID
FROM
(
      SELECT user AS ID FROM friends WHERE friend = 1
      UNION ALL 
      SELECT friend     FROM friends WHERE user = 1
) t 
GROUP BY ID
HAVING COUNT(ID) > 1;

SQL Fiddle Demo

This will give you:

| ID |
------
|  2 |
| 16 |