Question

我正在构建一个允许用户互相推荐音乐的应用程序，并且在构建查询时遇到问题，该查询会返回涉及用户自己以及任何用户朋友的推荐“流” 。这是我的表结构：

建议

ID     Sender     Recipient    [other columns...]
--     ------     ---------    ------------------
r1     u1         u3           ...
r2     u3         u2           ...
r3     u4         u3           ...

用户

ID      Email      First Name   Last Name     [other columns...]
---     -----      ----------   ---------     ------------------
u1      ...        ...          ...           ...
u2      ...        ...          ...           ...
u3      ...        ...          ...           ...
u4      ...        ...          ...           ...

关系

ID      Sender     Recipient     Status       [other columns...]
---     ------     ---------     --------     ------------------
rl1     u1         u2            accepted     ...
rl2     u3         u1            accepted     ...
rl3     u1         u4            accepted     ...
rl4     u3         u2            accepted     ...

因此，对于用户'u4'（谁是'u1'的朋友），我想查询与u4相关的推荐“流”。此流将包括发件人或收件人为u4的所有建议，以及发件人或收件人为u1（朋友）的所有建议。

这是我到目前为止的查询：

 SELECT * FROM recommendations
  WHERE recommendations.sender IN 
    ( SELECT sender FROM relationships WHERE recipient='u4' AND status='accepted' 
      UNION 
      SELECT recipient FROM relationships WHERE sender='u4' AND status='accepted') 
 OR recommendations.recipient IN 
    ( SELECT sender FROM relationships WHERE recipient='u4' AND status='accepted' 
      UNION 
      SELECT recipient FROM relationships WHERE sender='u4' AND status='accepted')
 UNION
 SELECT * FROM recommendations
 WHERE recommendations.sender='u4' OR recommendations.recipient='u4'
 GROUP BY recommendations.id
 ORDER BY datecreated DESC

据我所知，这似乎有用（我不是SQL专家）。它返回Recommendations表中与给定用户“相关”的所有记录。但是，我现在也无法从Users表中获取数据。 Recommendations表有发件人和收件人的ID（外键），但我也想得到每个人的名字和姓氏。我想我需要某种加入，但我迷失了如何继续，并正在寻求帮助。（而且，如果有人在我当前的查询中看到任何需要改进的地方，我都会听到。）

谢谢！

Answer 1

好的，相信我弄清楚了。必须创建2个users表的别名，以便我可以从每个表中拉出来。这是工作查询：

SELECT recommendations.*, user1.firstname AS 'senderFirstName', user1.lastname AS 'senderLastName', user2.firstname AS 'recipientFirstName', user2.lastname AS 'recipientLastName'
FROM (recommendations, users)
INNER JOIN users AS user1 ON (user1.id=recommendations.sender)
INNER JOIN users AS user2 ON (user2.id=recommendations.recipient)
WHERE recommendations.sender IN (SELECT sender FROM relationships WHERE recipient='4' AND status='accepted' UNION SELECT recipient FROM relationships WHERE sender='4' AND status='accepted') OR recommendations.recipient IN (SELECT sender FROM relationships WHERE recipient='4' AND status='accepted' UNION SELECT recipient FROM relationships WHERE sender='4' AND status='accepted')
UNION
SELECT recommendations.*, user1.firstname AS 'senderFirstName', user1.lastname AS 'senderLastName', user2.firstname AS 'recipientFirstName', user2.lastname AS 'recipientLastName'
FROM (recommendations, users)
INNER JOIN users AS user1 ON (user1.id=recommendations.sender)
INNER JOIN users AS user2 ON (user2.id=recommendations.recipient)
WHERE recommendations.sender='4' OR recommendations.recipient='4'
GROUP BY recommendations.id
ORDER BY date created DESC

如果有人发现任何错误或有改进的余地，我会感激任何建议。

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