Question

我正在尝试运行此查询，但它只输出包含项目的类别的结果，而不是所有类别。这是我的问题：

SELECT categories.`id` as `cat_id`, categories.`title` as `cat_title`, COUNT(tutorials.`id`) as `total`
FROM `tutorial_categories` as `categories`, `tutorials`
WHERE tutorials.`category` = categories.id
GROUP BY `cat_id`

有人能引导我朝着正确的方向前进吗？感谢。

Answer 1

SELECT 
categories.id , 
categories.title,
COUNT(*) as total
FROM tutorial_categories as categories
LEFT JOIN tutorials
on tutorials.category = categories.id
GROUP BY categories.id

Answer 2

您需要使用LEFT JOIN。您的隐式连接语法会生成内部联接insetad：

SELECT 
  categories.`id` as `cat_id`, 
  categories.`title` as `cat_title`, 
  COUNT(tutorials.`id`) as `total`
FROM
  `tutorial_categories` `categories` LEFT JOIN `tutorials`  ON categories.id = tutorials.category
GROUP BY `cat_id`

Answer 3

您正在寻找here所述的外部联接：“外部联接不要求两个连接表中的每条记录都有匹配的记录”

Answer 4

您需要使用外部联接重新设置查询，而不是使用WHERE

像这样的东西

SELECT categories.`id` as `cat_id`, categories.`title` as `cat_title`, COUNT(tutorials.`id`) as `total`
FROM `tutorial_categories` as `categories`, `tutorials`
LEFT OUTER JOIN tutorials.`category` = categories.id
GROUP BY `cat_id`

Answer 5

您设置了WHERE tutorials.类别= categories.id，因此它只会返回符合该条件的行。删除它，它应该返回“所有类别”。

SELECT categories.`id` as `cat_id`, categories.`title` as `cat_title`, COUNT(tutorials.`id`) as `total`
FROM `tutorial_categories` as `categories`, `tutorials`
GROUP BY `cat_id`

MySQL查询输出

5 个答案: