Question

我有一个包含类别和主题的网站。有些类别是彼此的父母/子女（只有一个嵌套级别）。我正在尝试从每个类别族中选择最近的主题（即，父母或孩子中最多只有一个主题）。这就是我现在正在尝试的事情：

SELECT topics.id
FROM topics, categories
WHERE topics.category_id = categories.id
AND ( categories.id IN ( 1, 2, 3 )
      OR categories.parent IN ( 1, 2, 3 ))
ORDER BY topics.posted DESC

如果按原样运行此查询，它将检索每个主题。如果我在末尾添加LIMIT 1，它只检索1个（不是每个家庭1个）。如果我在最后添加LIMIT 3，它将从所有类别中检索最近的3个。我想要每个类别的一个主题。有什么帮助吗？

Answer 1

有点粗糙，但应该做你想做的事：

SELECT topics.id FROM
  (SELECT MAX(topics.posted) AS date, categories.id AS id
  FROM topics, categories
  WHERE topics.category_id = categories.id
  AND ( categories.id IN ( 1, 2, 3 )
      OR categories.parent IN ( 1, 2, 3 ))
  GROUP BY categories.id) t1
JOIN topics 
ON topics.posted = t1.date AND topics.category_id = ti.id

Answer 2

我相信您正在寻找GROUP BY：

SELECT topics.id,MAX(topics.posted) AS postDate
FROM topics, categories
WHERE topics.category_id = categories.id
AND ( categories.id IN ( 1, 2, 3 )      
OR categories.parent IN ( 1, 2, 3 ))
GROUP BY topics.category_id
ORDER BY topics.posted DESC

次要的限制

2 个答案: