这两个表来自一个类似twitter的数据库，用户可以在其中跟踪其他用户。user.name字段是唯一的。

mysql> select uID, name from User;
+-----+-------------------+
| uID | name              |
+-----+-------------------+
|   1 | Alice             |
|   2 | Bob               |
|   5 | Iron Maiden       |
|   4 | Judas Priest      |
|   6 | Lesser Known Band |
|   3 | Metallica         |
+-----+-------------------+
6 rows in set (0.00 sec)

mysql> select * from Follower;
+-----------+------------+
| subjectID | observerID |
+-----------+------------+
|         3 |          1 |
|         4 |          1 |
|         5 |          1 |
|         6 |          1 |
|         3 |          2 |
|         4 |          2 |
|         5 |          2 |
+-----------+------------+
7 rows in set (0.00 sec)

mysql> call newFollowSuggestionsForName('Bob');
+-------------------+
| name              |
+-------------------+
| Lesser Known Band |
+-------------------+
1 row in set (0.00 sec)

我想做一个操作，为用户x建议一个他们可能感兴趣的用户列表。我认为一个启发式的方法是为所有y用户y显示x，其中x和y至少跟随3个相同的用户。下面是我为此提出的sql。我的问题是，在其他方面是否可以做得更有效或更好。

DELIMITER //
CREATE PROCEDURE newFollowSuggestionsForName(IN in_name CHAR(60))
BEGIN

DECLARE xuid INT;
SET xuid = (select uID from User where name=in_name);  

select name
from User, (select subjectID
            from follower
            where observerID in (
                select observerID
                from Follower
                where observerID<>xuid and subjectID in (select subjectID from Follower where observerID=xuid)
                group by observerID
                having count(*)>=3
            )
    ) as T
where uID = T.subjectID and not exists (select * from Follower where subjectID=T.subjectID and observerID=xuid);

END //
DELIMITER ;