MySQL选择查询不起作用

Question

I have 2 tables and I want to fetch users data from it.

Table 1 : frei_session

username    session_id

Prince      51
Dilip       49
Sumit       50

Table 2 : friendrequest

requestTo   requestFrom     status

49              50              C
50              51              C
49              51              P

friendrequest table show relationship between users where C stands for Complete and P stands for Pending . means if status = C both are friends and if status = P request is pending.

frei_session table display all online users of website.

I want to limit chat with friends only. I have tried following query but it shows all users which are online

suppose user 49 is online

SELECT DISTINCT f.username,f.session_id
    FROM frei_session AS f
    LEFT JOIN friendrequest AS fr ON fr.requestFrom=f.session_id
    LEFT JOIN friendrequest AS frnd ON frnd.requestTo=f.session_id
    WHERE (fr.requestFrom<>49 OR frnd.requestTo<>49) AND (fr.status = 'C' OR frnd.status = 'C')

This query show me output like this

username    session_id

Prince          51
Sumit           50

but I want output like this

username    session_id

Sumit       50

because user 51 and user 49 are not friends

Answer 1

SELECT  IF(a.RequestTo = 49, c.userName, b.userName) username,
        IF(a.RequestTo = 49, c.session_ID, b.session_ID) Session_ID
FROM    friendRequest a
        INNER JOIN frei_session b
            ON a.requestTo = b.session_ID
        INNER JOIN frei_session c
            ON a.requestFrom = c.session_ID
WHERE   a.status = 'C' AND
        49 IN (a.requestTo, a.requestFrom)

• SQLFiddle Demo
• SQLFiddle Demo ( when all in the friendrequest has status of C )

RESULT

╔══════════╦════════════╗
║ USERNAME ║ SESSION_ID ║
╠══════════╬════════════╣
║ Sumit    ║         50 ║
╚══════════╩════════════╝

UPDATE 1

DELIMITER $$
CREATE PROCEDURE ShowAllUserName(IN _sessionID INT)
BEGIN
    SELECT  IF(a.RequestTo = _sessionID, c.userName, b.userName) username,
            IF(a.RequestTo = _sessionID, c.session_ID, b.session_ID) Session_ID
    FROM    friendRequest a
            INNER JOIN frei_session b
                ON a.requestTo = b.session_ID
            INNER JOIN frei_session c
                ON a.requestFrom = c.session_ID
    WHERE   a.status = 'C' AND
            _sessionID IN (a.requestTo, a.requestFrom);
END;
DELIMITER $$

when calling the procedure,

CALL ShowAllUserName(49);

Answer 2

Why do you join twice the same table ?

try

LEFT JOIN friendrequest AS fr ON fr.requestFrom=f.session_id AND fr.requestTo=f.session_id

Than, you could change your where query by :

WHERE fr.requestFrom<>49 AND fr.status = 'C'

I'd try GROUP BY fr.status if I did understand your problem, it may work.

Answer 3

Try to use this query:

SELECT DISTINCT f.username,f.session_id
FROM frei_session AS f
LEFT JOIN friendrequest AS fr ON f.session_id IN (fr.requestFrom, fr.requestTo)
WHERE fr.requestFrom = 49 AND fr.status = 'C'

Answer 4

You can first get the users who have received request from Dilip. You then get the users who have sent request to Dilip. After that you union these users. The SQL is something like:

SELECT fs.username, fs.session_id
FROM   frei_session AS fs, friendrequest AS fr
WHERE  fs.username = "Dilip"
  AND  fs.session_id = fr.requestFrom
  AND  fr.status = "C"
UNION
SELECT fs.username, fs.session_id
FROM   frei_session AS fs, friendrequest AS fr
WHERE  fs.username = "Dilip"
  AND  fs.session_id = fr.requestTo
  AND  fr.status = "C"

I believe that this solution is easier to understand.

Answer 5

SELECT  IF(a.RequestTo = 49, c.userName, b.userName) username,
        IF(a.RequestTo = 49, c.session_ID, b.session_ID) Session_ID
FROM    friendRequest a
        INNER JOIN frei_session b
            ON a.requestTo = b.session_ID
        INNER JOIN frei_session c

            ON a.requestFrom = c.session_ID
WHERE   a.status = 'C' AND
        49 IN (a.requestTo, a.requestFrom)