MySQL选择查询不起作用

Question

我有2个表，我想从中获取用户数据。

表1： frei_session

username    session_id

Prince      51
Dilip       49
Sumit       50

表2： friendrequest

requestTo   requestFrom     status

49              50              C
50              51              C
49              51              P

friendrequest表显示用户之间的关系，其中C代表完成 ， P代表待定 。 表示如果status = C都是朋友，并且status = P请求正在等待。

frei_session表显示网站的所有在线用户。

我想限制与朋友聊天。 我尝试了以下查询，但它显示了所有在线用户

假设用户49在线

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')

这个查询显示我这样的输出

username    session_id

Prince          51
Sumit           50

但我想要这样的输出

username    session_id

Sumit       50

因为用户51和用户49不是朋友

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演示
• SQLFiddle演示（ 当所有的friendrequest具有的地位C ）

结果

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

更新1

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

在调用程序时，

CALL ShowAllUserName(49);

Answer 2

为什么你加入同一张桌子两次？

尝试

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

比，您可以通过以下方式更改您的查询位置：

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

如果我理解你的问题，我会尝试GROUP BY fr.status ，它可能会有效。

Answer 3

尝试使用此查询：

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

您可以先获得收到Dilip请求的用户。 然后，您将获得向Dilip发送请求的用户。 之后，您将这些用户联合起来。 SQL类似于：

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"

我相信这个解决方案更容易理解。

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)