在 MYSQL 中使用 GROUP_CONCAT 的问题

Question

我有以下表格：

帖子：id、标题、日期、内容

标签：id、名称

posts_tags：post_id、tag_id

我想选择带有相关标签列表的标题；以下查询效果很好

SELECT p.title, GROUP_CONCAT( t.name ORDER BY t.name SEPARATOR ',' ) as tags_list
FROM posts AS p
JOIN posts_tags AS pt ON pt.post_id = p.id
JOIN tags AS t ON pt.tag_id = t.id
GROUP BY p.id

通过这个查询，我得到了一些看起来像这样的东西

title             |  tags_list
==================|===============================
Old cinema        |  film,movies
cooking shows     |  cooking,food,kitchen,television
Epic War Films    |  history,movies,war
Art in France     |  art,france,history

当我只想发布带有特定标签的帖子（例如“电影”）时，我尝试添加 where 子句：

SELECT p.title, GROUP_CONCAT( t.name ORDER BY t.name SEPARATOR ',' ) as tags_list
FROM posts AS p
JOIN posts_tags AS pt ON pt.post_id = p.id
JOIN tags AS t ON pt.tag_id = t.id
WHERE t.name = 'movies'
GROUP BY p.id

我最终得到了这个

title            |  tags_list
=================|===============================
Old cinema       |  movies
Epic War Films   |  movies

我得到了所有标记为“电影”的帖子，但问题是“tags_list”列仅在每一行中显示“电影”，而不包括与帖子关联的所有其他标签。

我该如何解决这个问题？

Answer 1

条件需要放在GROUP_CONCAT 的结果上而不是每一行上，所以@AshwinMukhija 建议条件需要放在HAVING 子句中：

SELECT 
    p.title, 
    GROUP_CONCAT( t.name ORDER BY t.name SEPARATOR ',' ) as tags_list
FROM 
    posts AS p
JOIN posts_tags AS pt 
    ON pt.post_id = p.id
JOIN tags AS t 
    ON pt.tag_id = t.id
GROUP BY 
    p.id
HAVING
    FIND_IN_SET('movies',tags_list)

如果在逗号分隔列表tags_list中找到字符串movies，FIND_IN_SET返回true

Answer 2

您只需将标签上的条件从where 子句转换为having

SELECT p.title, GROUP_CONCAT( t.name ORDER BY t.name SEPARATOR ',' ) as tags_list
FROM posts AS p
JOIN posts_tags AS pt ON pt.post_id = p.id
JOIN tags AS t ON pt.tag_id = t.id
GROUP BY p.id
HAVING t.name = 'movies'