獲取所有從另一個

Question

ķ算的，所以我有兩個表：

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 | 
+------------------+-------------+ 

我想從類別獲得所有和計數對每一類問題（question_id）的數量。

假設第一類帳單有1個問題，第二個帳號有3個問題。

我已經試過這樣：

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

Answer 1

你想這樣做：

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 

該LEFT JOIN將確保沒有問題的類別被列出與計數= 0

Answer 2

這應該這樣做......你只需要計數之前的東西彙總：

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 =「從CustomerTable c，OrderTable中選擇c。*，count（o.id）o其中c.id = o.customerId GROUP BY c.id」;

希望這會幫助你。謝謝