在一个查询中联接/查找MySQL三个表

Question

I decided to create People You May Know System but i have accounted a problem where it seems I need to join three tables to bring relevant results.

The aim is to check in database table for possible friends one may know based on friend of my friend (if user1 = user2 and user2 = user4 posibile user1 knows user4, user4 is a suggestion) I have used the following code to check this and is working as to plan.

SELECT `friendpin` AS `possible_friend_id`
FROM `pinme`
WHERE `senderpin`
IN (
SELECT `friendpin`
FROM `pinme`
WHERE `senderpin` = $userlogged
)
AND `friendpin` NOT
IN (

SELECT `friendpin`
FROM `pinme`
WHERE `senderpin` = $userlogged
)
AND NOT `friendpin` = $userlogged
GROUP BY `possible_friend_id`
ORDER BY COUNT( * ) DESC
LIMIT 0 , 3 

Thus its possible that the suggested person status in REQUEST table he/she has already a request pending to you but still being shown as suggested friend or in the BLOCKED table the suggested friend was blocked but still appear to you as suggested friend. therefore I would like to eliminate this behavior. a suggested person should not be exits either in REQUEST table or BLOCKED table.

Can anyone help me solve this?

the following are the REQUEST table, PINME table and BLOCKED table structure

REQUEST TABLE:
senderpin - id of the person who send request   
friendpin - receivers request id    


BLOCKED TABLE: 
user1_id - a person who block user2_id
user2_id - the blocked person by user1_id


PINME TABLE:
PINME table: the table which keep relations
structure: 1,2 2,2 | 4,8 8,4
senderpin - id of the person who send request   
friendpin - receivers request id

Thanks and regards

Answer 1

What you seem to need to do is join the pinme table against itself, and then against itself again to find the matches. Do a LEFT OUTER JOIN with the results of that against the blocked table and against the request table to exclude any records where there is already a blocked or request record.

Something like this:-

SELECT DISTINCT c.aPerson
FROM (SELECT friendpin AS aFriend FROM pinme WHERE senderpin = $userlogged UNION SELECT senderpin AS aFriend FROM pinme WHERE friendpin = $userlogged) a
INNER JOIN (SELECT senderpin AS aPerson, friendpin AS aFriend FROM pinme UNION SELECT senderpin AS aFriend, friendpin AS aPerson FROM pinme) b
ON a.aFriend = b.aFriend
INNER JOIN (SELECT senderpin AS aPerson, friendpin AS aFriend FROM pinme UNION SELECT senderpin AS aFriend, friendpin AS aPerson FROM pinme) c
ON b.aPerson = c.aFriend
LEFT OUTER JOIN (SELECT user1_id AS blocked FROM blocked WHERE user2_id = $userlogged UNION SELECT user2_id AS aFriend FROM blocked WHERE user1_id = $userlogged) d
ON c.aPerson = d.blocked
LEFT OUTER JOIN (SELECT senderpin AS blocked FROM request WHERE friendpin = $userlogged UNION SELECT friendpin AS aFriend FROM request WHERE senderpin = $userlogged) e
ON c.aPerson = e.blocked
WHERE d.blocked IS NULL
AND  e.blocked IS NULL
AND c.aPerson != $userlogged

EDIT - Modified suggestion

SELECT DISTINCT c.aPerson
FROM (SELECT senderpin AS aPerson, friendpin AS aFriend FROM pinme WHERE senderpin = $userlogged UNION SELECT senderpin AS aFriend, friendpin AS aPerson FROM pinme WHERE friendpin = $userlogged) a -- Find friends
INNER JOIN (SELECT senderpin AS aPerson, friendpin AS aFriend FROM pinme UNION SELECT senderpin AS aFriend, friendpin AS aPerson FROM pinme) b -- Find friends of friends who are not the original person
ON a.aFriend = b.aFriend AND a.aPerson != b.aFriend
LEFT OUTER JOIN (SELECT senderpin AS aPerson, friendpin AS aFriend FROM pinme WHERE senderpin = $userlogged UNION SELECT senderpin AS aFriend, friendpin AS aPerson FROM pinme WHERE friendpin = $userlogged) c -- Find friends
ON c.aFriend = b.aPerson 
LEFT OUTER JOIN (SELECT user1_id AS blocked FROM blocked WHERE user2_id = $userlogged UNION SELECT user2_id AS aFriend FROM blocked WHERE user1_id = $userlogged) d
ON b.aPerson = d.blocked
LEFT OUTER JOIN (SELECT senderpin AS blocked FROM request WHERE friendpin = $userlogged UNION SELECT friendpin AS aFriend FROM request WHERE senderpin = $userlogged) e
ON b.aPerson = e.blocked
WHERE d.blocked IS NULL
AND  e.blocked IS NULL
AND  c.aFriend IS NULL
AND b.aPerson != $userlogged