多对多查询标记对象

Question

我有这个数据库架构。我只是为了简单起见而调用我标记对象的东西：

用户

• user_id
• 登录

用户到对象表（多对多）

• user_id
• object_id

对象表

• object_id
• object_stuff

用 user_id 表标记的对象（多对多带有额外的列）

• user_id
• object_id
• tag_id

标签表

• tag_id
• 标签名称

我一直在使用这样的查询来获取特定用户的对象以及对象的标签（如果用户标记了任何标签）。 如果您认为可以对此进行优化，请随时告诉我。

@user_id = 'whatever user_id I want';
SELECT 
    o.object_id AS object_id,
    o.object_stuff AS object_stuff,
    IF (tags.tag_name IS NOT NULL, GROUP_CONCAT(tags.tag_name SEPARATOR '|'), 'N/A') AS tags
FROM  object_table AS o
LEFT OUTER JOIN  obj_to_users_table AS o2u ON o.object_id = o2u.object_id
LEFT OUTER JOIN  users AS u ON u.user_id = o2u.user_id 
LEFT OUTER JOIN  obj_to_tag_users AS o2tu ON o.object_id = o2tu.object_id AND u.user_id = o2tu.user_id
LEFT OUTER JOIN  tags AS t ON t.tag_id = o2t.tag_id 
WHERE u.user_id = @user_id GROUP BY o.object_id

这会产生如下结果：

object_id     object_stuff        tags
1             stuff1...           tag1|tag2|tag3
2             stuff2...           tag4|tag6|tag1
3             stuff3...           tag7|tag2|tag5

我的问题是，例如，我想搜索标记为“tag2”的对象，我应该得到：

object_id     object_stuff        tags
1             stuff1...           tag1|tag2|tag3
3             stuff3...           tag4|tag2|tag1

但相反，我丢失了该对象附带的其他标签：

object_id     object_stuff        tags
1             stuff1...           tag2
3             stuff3...           tag2

如何修改我的 WHERE 子句，以便获得标记为“tag2”的对象，同时在结果集中保留该对象的其他标签？

... WHERE u.user_id = @user_id AND t.tag_name LIKE '%tag2%' ...

Answer 1

首先，计算级联结果。

接下来，针对这些连接的结果执行搜索以保留标签的分组：

SELECT * 
FROM
(
    SELECT 
        o.object_id AS object_id,
        o.object_stuff AS object_stuff,
        IF (tags.tag_name IS NOT NULL, 
            GROUP_CONCAT(tags.tag_name SEPARATOR '|'), 'N/A') AS tags
    FROM  object_table AS o
    LEFT OUTER JOIN  obj_to_users_table AS o2u 
        ON o.object_id = o2u.object_id
    LEFT OUTER JOIN  users AS u 
        ON u.user_id = o2u.user_id 
    LEFT OUTER JOIN  obj_to_tag_users AS o2tu 
        ON o.object_id = o2tu.object_id AND u.user_id = o2tu.user_id
    LEFT OUTER JOIN  tags AS t ON t.tag_id = o2t.tag_id 
    WHERE u.user_id = @user_id 
    GROUP BY o.object_id
) sub
WHERE sub.tags LIKE '%tag2%'

编辑：

---

这是一个使用EXISTS 的示例。我还将LEFT JOINs 切换为INNER JOINs，因为搜索特定标签应该消除任何objects 而没有tags。

虽然这个查询看起来更复杂，但它应该更有可能使用任何可用的索引。

我还提到过使用一组额外的INNER JOINs 执行相同类型的查询，但是那个看起来会更复杂，因为您仍然必须使用子查询来使用DISTINCT 来获取消除额外的JOINs 引入的任何可能的重复项...所以我建议改用EXISTS...

SELECT 
    o.object_id AS object_id,
    o.object_stuff AS object_stuff,
    GROUP_CONCAT(t.tag_name SEPARATOR '|') AS tags
FROM  object_table AS o
INNER JOIN  obj_to_users_table AS o2u 
    ON o.object_id = o2u.object_id
INNER JOIN  users AS u 
    ON u.user_id = o2u.user_id 
INNER JOIN  obj_to_tag_users AS o2tu 
    ON o.object_id = o2tu.object_id AND u.user_id = o2tu.user_id
INNER JOIN tags AS t 
    ON t.tag_id = o2t.tag_id
WHERE 
    u.user_id = @user_id 
    AND EXISTS (
        SELECT 1
        FROM tags AS targetTag
        WHERE 
            targetTag.tag_id = o2t.tag_id
            AND targetTag.tag_name = 'tag2'
    )
GROUP BY o.object_id