Question

K，所以我有两张桌子：

categories
+----+----------+
| id | slug     |
+----+----------+
| 1  | billing  |
| 2  | security |
| 3  | people   |
| 4  | privacy  |
| 5  | messages |
+----+----------+

categories_questions
+------------------+-------------+
| id | question_id | category_id |
+------------------+-------------+
| 1  |           1 |           2 |
| 2  |           2 |           5 |
| 3  |           3 |           2 |
| 4  |           4 |           4 |
| 5  |           4 |           2 |
| 6  |           5 |           4 |
+------------------+-------------+

我希望从类别中获取所有内容并计算每个类别的问题数量（question_id）。

比方说，第一类，结算，将有1个问题，第二类，安全，将有3个问题。

我试过这个：

SELECT categories.*, count(categories_questions.id) AS numberOfQuestions
FROM categories
INNER JOIN categories_questions
ON categories.id = categories_questions.category_id

Answer 1

你想这样做：

SELECT categories.id, max(categories.slug), count(categories_questions.id) AS numberOfQuestions
FROM categories
LEFT JOIN categories_questions
ON categories.id = categories_questions.category_id
group by categories.id

LEFT JOIN将确保没有问题的类别列在count = 0

中

Answer 2

这应该这样做......你只需要在计算之前聚合一些东西：

SELECT categories.*, COUNT(categories_questions.id) AS numberOfQuestions FROM categories
INNER JOIN categories_questions
ON categories.id = categories_questions.category_id
GROUP BY categories.id

Answer 3

String p_query =＆＃34;从CustomerTable c，OrderTable o中选择c。*，count（o.id），其中c.id = o.customerId GROUP BY c.id＆＃34 ;;

希望这会对你有所帮助。谢谢

从一个表中获取所有内容，从另一个表中获取COUNT

3 个答案: