根据没有联合的值，改进查询以联接其他表

Question

我有一个包含以下数据的表：

+-----------------+--------------+----------------------+------------+--------------------+-----------+------+---------+---------+-----+------------------------+---------------------+
| notification_id | from_user_id | from_user_auth_level | to_user_id | to_user_auth_level | status_id | type | subject | message | url | timestamp_inserted_utc | timestamp_read_utc  |
+-----------------+--------------+----------------------+------------+--------------------+-----------+------+---------+---------+-----+------------------------+---------------------+
|               1 | NULL         | NULL                 |          1 |                  1 |         0 |    2 | test    | test    | url | 2010-10-10 00:00:00    | 2011-10-10 00:00:00 |
|               2 | 2            | 5                    |          1 |                  1 |         0 |    2 | test    | test    | url | 2010-10-10 00:00:00    | 2011-10-10 00:00:00 |
|               3 | 3            | 5                    |          1 |                  1 |         0 |    2 | test    | test    | url | 2010-10-10 00:00:00    | 2011-10-10 00:00:00 |
|               4 | 2295         | 4                    |          1 |                  1 |         0 |    2 | test    | test    | url | 2010-10-10 00:00:00    | 2011-10-10 00:00:00 |
|               5 | 10           | 1                    |          1 |                  1 |         0 |    2 | test    | test    | url | 2010-10-10 00:00:00    | 2011-10-10 00:00:00 |
+-----------------+--------------+----------------------+------------+--------------------+-----------+------+---------+---------+-----+------------------------+---------------------+

然后我还有其他一些表，例如'users'，'companies'，'organizations ...等。>

我需要能够获取每个通知的用户名，性别和图像（基于from_user_id和from_user_auth_level）。

但是问题在于事实，该信息位于不同的位置，具体取决于user_auth_level是什么。

例如：如果我的用户是“常规”用户，则他的auth_level将为1。该图像将驻留在我的“用户”表中，并且性别适用。但是，如果用户的auth_level == 5，则表示他是一个组织。在这种情况下，性别不适用，并且图像位于“组织”表中，需要通过用户将其链接到user_roles，然后再链接到组织。

而且对于每种用户类型，它们都需要不同的联接。

我创建了一个有效的查询，但是它在所有地方都使用UNION，并且我已经读到它并不是出于性能原因的最佳选择，所以我希望有人可以指导我在性能上改进此查询：] >

                SELECT n.*, NULL as username, NULL as gender, NULL as picture
                FROM notification as n
                WHERE n.from_user_auth_level IS NULL
                AND n.to_user_id = $userid
                UNION
                SELECT n.*, u.username, u.gender as gender, u.profile_picture as picture
                FROM notification as n
                LEFT JOIN users AS u ON n.from_user_id = u.user_id
                WHERE n.from_user_auth_level = 1
                AND n.to_user_id = $userid
                UNION
                SELECT n.*, u.username, NULL as gender, c.logo as picture
                FROM notification as n
                LEFT JOIN users AS u ON n.from_user_id = u.user_id
                LEFT JOIN user_companies AS uc on u.user_id = uc.user_id
                LEFT JOIN company as c on uc.company_id = c.company_id
                WHERE n.from_user_auth_level = 4
                AND n.to_user_id = $userid
                UNION
                SELECT n.*, u.username, NULL as gender, o.logo as picture
                FROM notification as n
                LEFT JOIN users AS u ON n.from_user_id = u.user_id
                LEFT JOIN user_roles as ur on u.user_id = ur.user_id
                LEFT JOIN organization as o on ur.org_id = o.org_id
                WHERE n.from_user_auth_level = 5
                AND n.to_user_id = $userid
                UNION
                SELECT n.*, u.username, NULL as gender, o.logo as picture
                FROM notification as n
                LEFT JOIN users AS u ON n.from_user_id = u.user_id
                LEFT JOIN user_roles as ur on u.user_id = ur.user_id
                LEFT JOIN organization as o on ur.org_id = o.org_id
                WHERE n.from_user_auth_level = 7
                AND n.to_user_id = $userid
                UNION
                SELECT n.*, u.username, NULL as gender, NULL as picture
                FROM notification as n
                LEFT JOIN users AS u ON n.from_user_id = u.user_id
                WHERE n.from_user_auth_level = 9
                AND n.to_user_id = $userid"
得到此结果后，我使用PHP根据timestamp_inserted_utc对结果进行排序，因为用UNION无法获得正确的结果。 

我有一个包含以下数据的表：+ ----------------- + -------------- + ----- ----------------- + ------------ + ------------------- -+ ----------- + ------ + --------- + --------- + ----- + --- ---------------...

Answer 1

示意性：

SELECT main.*,
       COALESCE(slave1.field, slave2.field, ...) AS field
       ...
FROM main
LEFT JOIN slave1 ON main.x=slave1.y AND main.slave = 1
LEFT JOIN slave2 ON main.x=slave2.z AND main.slave = 2
...

Answer 2

我将通知表用作基础，并将条件外部联接用作：