我怎樣才能做一個查詢選擇嵌套？

Question

我有兩個表：

// users 
+----+--------+ 
| id | name | 
+----+--------+ 
| 1 | Jack | 
| 2 | Peter | 
| 3 | John | 
| 4 | Barman | 
| 5 | Ali | 
+----+--------+ 

// friends 
+---------+-----------+ 
| user_id | friend_id | 
+---------+-----------+ 
| 1  | 3   | 
| 1  | 4   | 
| 1  | 5   | 
| 3  | 1   | 
| 3  | 2   | 
| 3  | 4   | 
| 5  | 2   | 
+---------+-----------+ 
-- both user_id and friend_id columns refer to the id column of users table 

我想選擇的Jack（id = 1）所有的朋友。所以這裏是查詢：

select * from friend where user_id = 1 
/* output 
| 1  | 3   | 
| 1  | 4   | 
| 1  | 5   | 
*/ 

現在我還想選擇朋友Jack的朋友。我怎樣才能做到這一點？

---

請注意，我不想選擇重複的行。所以，我想這樣的輸出：

/* expected output: 
| 1  | 3   | 
| 1  | 4   | 
| 1  | 5   | 
| 3  | 2   | 
| 3  | 4   | 
| 5  | 2   | 
*/ 

Answer 1

添加IN子句與傑克的所有朋友使用不同的USER_ID，friend_id

select distinct f1.user_id, f1.friend_id 
from friend f1 
where user_id = 1 
     or 
     user_id in (select f2.friend_id 
        from friend f2 
        where user_id = 1); 

Answer 2

select distinct 
    f2.* 
from 
    friend f1, 
    friend f2 
where 
    f1.user_id = 1 and 
    (f1.friend_id = f2.user_id or f2.user_id = 1) 

這仍然包括重複與相反方向（它認爲A--朋友 - > B與B不同 - 朋友 - > A）