如何使用两个联接

Question

我有一个聊天表，在user_from和user_to中有用户表使用的引用ID。 现在我需要查询从facebook caht list这样的聊天表中获取聊天成员列表。 我和谁聊天，我必须从用户表和未读计数器中找到用户名

请给我解决方案 感谢

Answer 1

我认为您的子查询需要相关，这将很慢。

改为使用JOIN。

SELECT 
    users.id AS userid,
    chat_box.id AS msgid,
    first_name,
    last_name,
    profile_pic,
    LEFT(message, 50) AS message,
    u.unreadcounter
FROM
    chat_box
        LEFT JOIN
    users ON (chat_box.user_from = users.id)
        LEFT JOIN
    (SELECT 
        u.id, COUNT(c.id) unreadcounter
    FROM
        chat_box c
    JOIN users u ON (c.user_from = u.id)
    WHERE
        c.read = 0 && c.user_to = 45
    GROUP BY u.id) u ON users.id = u.id
WHERE
    (user_from = 45 OR user_to = 45)
        && users.id != 45
GROUP BY users.id 
UNION SELECT 
    users.id AS userid,
    chat.id AS msgid,
    first_name,
    last_name,
    profile_pic,
    LEFT(message, 50) AS message,
    0 AS unreadcounter
FROM
    chat_box AS chat
        LEFT JOIN
    users ON (chat.user_to = users.id)
WHERE
    (user_from = 45 OR user_to = 45)
        && users.id != 45
GROUP BY users.id
ORDER BY msgid DESC