如果連接表中不存在，則顯示0的SQL命令

Question

我在SQL命令中遇到問題。

我有一個問題表，其他問題可能的答案和其他與用戶的答覆。

想象一下下面的例子：

問題1：誰將贏得半決賽？
Aswners：A）葡萄牙B）西班牙
回覆：10人投B）西班牙，0人投票A）葡萄牙

SELECT a.answer, COUNT(r.id) as total 
FROM replies r 
LEFT JOIN answers a ON a.id = r.id_answer 
LEFT JOIN questions q ON q.id = a.id_question 
WHERE q.id = 1 
GROUP BY r.id_answer 

我的觀點是從
SELECT結果得到：
西班牙10
葡萄牙0

但是我不能，我不知道該怎麼做，因爲我這樣做，我總是從答覆表上得到答案的結果。就像這樣：
西班牙10

Answer 1

你將不得不從你的問題開始，並LEFT JOIN的答覆。

SELECT a.answer, COUNT(r.id_answer) AS total 
FROM questions q 
JOIN answers a ON (a.id_question = q.id) 
LEFT JOIN replies r ON (r.id_answer = a.id) 
WHERE q.id = 1 
GROUP BY a.id, a.answer 

根據您目前的查詢，你甚至都不需要question：

SELECT a.answer, COUNT(r.id_answer) AS total 
FROM answers a 
LEFT JOIN replies r ON (r.id_answer = a.id) 
WHERE a.id_question = 1 
GROUP BY a.id, a.answer 

See this example爲您的SQL小提琴第二個查詢，返回：

ANSWER  TOTAL 
Spain  10 
Portugal 0 

Answer 2

SELECT a.answer, COUNT(r.id) as total 
FROM questions q 
INNER JOIN answers a ON a.id_question = q.id 
LEFT JOIN replies r ON r.id_answer = a.id 
WHERE q.id = 1 
GROUP BY a.id; 

使用LEFT JOIN代替內連接。

編輯：加入更新。

Answer 3

您的查詢應該是這樣 -

SELECT a.answer, COUNT(r.id) as total 
FROM questions q 
INNER JOIN JOIN answers a ON q.id = a.id_question 
LEFT JOIN REPLIES r ON a.id = r.id_answer 
WHERE 
q.id=1 
GROUP BY 
a.answer 

這裏最重要的是count(r.id)得到Portugal 0