/*Your friends' friends*/
SELECT r2.from_id FROM
table_relations r
INNER JOIN table_relations r2
ON r.from_id = r2.to_id
WHERE r.to_id = 1
EXCEPT /*EXCEPT is Standard but MINUS in Oracle*/
/*Your friends*/
SELECT from_id
FROM table_relations
WHERE to_id = 1

或者使用OUTER JOIN

SELECT DISTINCT r2.from_id
FROM
table_relations r
INNER JOIN table_relations r2
ON r.from_id = r2.to_id
LEFT OUTER JOIN table_relations r3 ON r3.to_id = r2.from_id AND r3.from_id=1
WHERE r.to_id = 1 AND r3.from_id is null