Question

我有两张桌子

类别：

cat_id，cat_name
主题：

topic_id，cat_id，topic_name

我想获取所有cat_names的列表以及主题表中针对特定cat_id的主题数量，例如

Category       Topics
----------------------
ABC            2
CDE            5

非常感谢快速帮助

由于

Answer 1

使用GROUP BY条款

SELECT COUNT(*) AS 'Topics', cat_id AS 'Category' FROM Category JOIN Topics ON Category.cat_id = Topics.cat_id
GROUP BY cat_id

Answer 2

尝试此查询

select a.cat_name as category, count(*) as Topics
from category a
  join Topics b on a.cat_id=b.cat_id
group by a.cat_name

Answer 3

var countSelected = _.filter(Options, { 'Selected': true});
console.log(countSelected.length);

Answer 4

试试这个

select cat_name, count(*) as counting
from Category as cat 
  inner join Topics as t on cat.cat_id=t.cat_id
group by cat_name

Answer 5

尝试这个

SELECT Category.cat_name as Category,
       (SELECT COUNT(topic_id) FROM  Topics
        WHERE Topics.cat_id = category.cat_id) AS Topics 
FROM Category

Answer 6

这是您在评论中显示的自己的查询：

select 
  category.cat_id as id, 
  category.cat_name as category, 
  count(select * from topics where topics.topic_id=id) as topics
from category 
inner join topics on category.cat_id=topics.cat_id;

首先是语法错误。而不是

count(select * from topics where topics.topic_id=id) as topics

这必须是

(select count(*) from topics where topics.topic_id=id) as topics

然后，您正在阅读主题表两次，一次在连接中，一次在子查询中。所以这看起来像是在一个声明中混合了两次尝试。以下是两个分开的选项：

select 
  category.cat_id as id, 
  category.cat_name as category, 
  (select count(*) from topics where topics.topic_id = category.id) as topics
from category;

或者：

select 
  category.cat_id as id, 
  category.cat_name as category, 
  count(topics.topic_id) as topics
from category 
left join topics on category.cat_id = topics.cat_id
group by category.cat_id;

使用计数编写SQL查询

6 个答案: