連接兩個表與conditon

Question

我有兩個表：

users 
- id 
- fname 
- lname 
- is_online 

和：

friendships 
- friendA (references users) 
- friendB (references users) 
- status (1 means they are friends) 

我想查詢一個給定的用戶誰是網上的所有朋友。

Answer 1

SELECT 
t1.* 
FROM users t1 
WHERE 
t1.id IN (
    SELECT 
    friendB 
    FROM friendships 
    WHERE 
    friendA = CURRENT_USER_ID 
    UNION 
    SELECT 
    friendA 
    FROM friendships 
    WHERE 
    friendB = CURRENT_USER_ID 
) 
t1.is_online = 1 
AND NOT t1.id = CURRENT_USER_ID 

Answer 2

Select 
    Unique(id) 
from 
    users outer join 
    friendships where 
     ((friendA = id and friendB = ID_TO_QUERY) or 
     (friendA = ID_TO_QUERY and friendB = id) and status = 1) 
     and is_online = true 
     and id /= ID_TO_QUERY