用条件连接两个表

Question

I have two tables:

users
- id
- fname
- lname
- is_online

and:

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

I want to query all friends of a given user who are online.

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