使用条件子句多连接mysql表

Question

我有三个表，我需要加入教育目的。

Table: users 
id | username | photo 
    1 | me  | my_photo 
    2 | my_friend| friend's photo 
    3 | somebody | somebody`s photo 

Table: chat 
mid | sender | receiver | created 
    1 | me  | my friend| 2016-12-27 09:32:2 
    2 | my_friend | me  | Y-m-d h:i:s 
    3 | me  | somebody | 2016-12-27 09:32:2 
    4 | somebody | me  | Y-m-d h:i:s 

Table:favorites 
    id | subscriber | subscribed_to 
    1 | me   | my_friend 

所需的输出是

Conversations: 
username | photo   | created   | subscribed 
me  | my friend's photo| 2016-12-27 09:32:2 | yes 
my_friend | my foto   | Y-m-d h:i:s  | no 
me  | somebody's foto | 2016-12-27 09:32:2 | no 
somebody | my foto   | Y-m-d h:i:s  | no 

目前我没有使用该代码的第一部分：

$qmessage = mysql_query(" 
select c.*, u.photo 
from chat c 
join users u on u.username = c.sender 
WHERE (sender = '$login_session' AND receiver = '$uid') OR 
     (sender = '$uid' AND receiver = '$login_session') 
"); 

Answer 1

我想了一系列的连接是你真正需要的在这里。下面查询中唯一可能需要解释的部分是最后的左连接。我使用发送者列和接收者列将chat表加入favorites表。然后我使用CASE表达式来检查给定记录是否匹配。如果是这样，那么它假定有订阅并且'yes'被打印，否则打印'no'。

SELECT t1.sender AS username, 
     COALESCE(t2.photo, 'NA') AS photo 
     t1.created, 
     CASE WHEN t3.subscriber IS NOT NULL THEN 'yes' ELSE 'no' END AS subscribed 
FROM chat t1 
LEFT JOIN users t2 
    ON t1.receiver = t2.username 
LEFT JOIN favorites t3 
    ON t1.sender = t3.subscriber AND 
     t1.receiver = t3.subscribed_to