Question

我有3个表（“新闻”，“类别”和连接表“meta”）。我只是想过滤包含category_id的新闻，该新闻作为过滤器参数提供。我尝试了下面的查询（和类似的），但它从新闻表中返回所有新闻。

那么如何按meta_id (category_id)过滤这些新闻？

非常感谢！

// category table
category_id | category_name
---------------------------
55          | Sport
56          | Politics

// meta table
meta_id | meta_type | rel_id | rel_type
---------------------------------------
55      | category  | 78345  | news
56      | category  | 91278  | news

Notes:
rel_id (in meta table)  = id (in news table) // i don't need it in this case
meta_id (in meta table) = category_id (in category table)

SQL：

SELECT n.*
FROM `news` n
LEFT JOIN `metas` m ON m.meta_type='category'
    WHERE m.meta_id=55 AND n.status=1
GROUP BY n.id
ORDER BY n.id DESC
LIMIT 0,10

Answer 1

最重要的是将metas与news相关联。如果我正确阅读了您的问题，则链接字段为m.rel_id = n.id。以下示例查询应筛选类别55：

select  *
from    metas m
join    news n
on      m.rel_id = n.id
where   m.meta_type = 'category'
        and m.meta_id = 55
        and n.status = 1
order by
        n.id desc
limit   0, 10

N.B。不确定为什么您的查询中有group by。当然不需要过滤类别！

Answer 2

您必须在ON子句中添加条件。

SELECT n.*
FROM `news` n
LEFT JOIN `metas` m ON m.meta_type='category' AND m.meta_id = n.category_id
    WHERE m.meta_id=55 AND n.status=1
GROUP BY n.id
ORDER BY n.id DESC
LIMIT 0,10

Answer 3

SELECT n.*
FROM `news` n
LEFT outer JOIN `metas` m ON m.meta_id = n.category_id
    WHERE m.meta_id=55 AND n.status=1 and m.meta_type='category'
GROUP BY n.id
ORDER BY n.id DESC
LIMIT 0,10

SQL元表帮助（mysql）

3 个答案: