mysql - 如何对 NULL 字段进行分组

Question

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，称为 thereIsANull
• 按 user category_id
字段对我的所有结果进行分组
• 如果字段用户(分组时)中没有 NULL，则将 thereIsANull 设置为 1，否则设置为 0。

因此在下面的示例中，结果必须是 1 行:

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

并且，如果所有用户都不为 NULL:

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

如何在 MySql 上实现？

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

如果没有 null，IsThereANull 将为 0，如果有，则为 1 或更多。