如何分组NULL字段

Question

在此查询之后：

SELECT forum_categories.id AS category_id, forum_categories.title, forum_topics.id AS topic_id, forum_topics.title, user
FROM forum_categories JOIN forum_topics ON forum_categories.id=forum_topics.category_id
LEFT OUTER JOIN forum_views ON forum_topics.id=forum_views.topic_id
WHERE forum_categories.id=6
ORDER BY forum_categories.id

我得到了这个结果：

现在，我想：

• 在选择中添加另一个字段，名为 thereIsANull
• 按字段 用户 category_id
对我的所有结果进行分组
• 如果字段用户中没有NULL（分组时），则将 thereIsANull 设置为1，否则为0。

所以在下面的例子中，结果必须是1行：

6    Welcome    (some topic_id)     (some title)   (djfonplaz or null)   0

并且，如果所有用户都与NULL不同：

6    Welcome    (some topic_id)     (some title)   (djfonplaz)   1

如何在MySql上执行此操作？

Answer 1

SELECT 
  forum_categories.id AS category_ids
  , forum_categories.title as titles
  , forum_topics.id AS topic_ids
  , forum_topics.title as topic_titles
  , count(*) as NumberOfRows
  , GROUP_CONCAT(IFNULL(user,'(no user)')) AS users 
  , (count(*) - count(user)) as IsThereANull  
FROM forum_categories 
INNER JOIN forum_topics ON forum_categories.id=forum_topics.category_id
LEFT OUTER JOIN forum_views ON forum_topics.id=forum_views.topic_id
WHERE forum_categories.id=6
GROUP BY category_id
ORDER BY forum_categories.id

IsThereANull将0没有空，如果有，则为1或更多。

Answer 2

我认为以下查询应该得到您正在寻找的结果：

SELECT forum_categories.id AS category_id, forum_categories.title, 
       forum_topics.id AS topic_id, forum_topics.title, user, 
       (
          SELECT MAX(IF(COUNT(*), 0, 1))
          FROM forum_categories JOIN forum_topics ON 
               forum_categories.id=forum_topics.category_id
          LEFT OUTER JOIN forum_views ON forum_topics.id=forum_views.topic_id
          WHERE forum_categories.id=6 AND user IS NULL
    ) AS thereIsANull
FROM forum_categories JOIN forum_topics ON forum_categories.id=forum_topics.category_id
LEFT OUTER JOIN forum_views ON forum_topics.id=forum_views.topic_id
WHERE forum_categories.id=6 AND user IS NOT NULL
GROUP BY user
ORDER BY forum_categories.id

Answer 3

我认为这样的事情可以按照你的意愿行事：

SELECT cat.id AS category_id, 
       cat.title, 
       top.id AS topic_id, 
       top.title, 
       user,
       (
           (
               SELECT COUNT(*) FROM forum_categories 
               WHERE id = cat.id AND user IS NULL
           ) > 0 -- This is to check wether if any NULL is found
       ) as thereIsANull 
FROM forum_categories as cat
JOIN forum_topics as top ON cat.id = top.category_id
LEFT OUTER JOIN forum_views as view ON top.id = view.topic_id
WHERE cat.id = 6 -- You can ofcourse change this to whatever variable your using
GROUP BY cat.id -- Just an ordinary group by category id
ORDER BY cat.id

假设在forum_categories表中找到了字段用户，那么只需修改子查询以加入从中获取用户的表

请注意，Thiis与Cez写的基本相同，所以他应该得到一些信用，唯一的区别是子查询和按类别ID而不是用户分组。

尝试一下，让我知道它是否正确：）