MySQL的JOIN沒有選擇的答案，如果沒有票

Question

看到這個代碼

SELECT x.*, z.rates 
FROM poll_answers x 
    JOIN 
    (SELECT answer_id, COUNT(id) AS `rates` 
    FROM poll_votes 
    GROUP BY answer_id) z 
    ON 
    x.id = z.answer_id 
WHERE x.poll_id = '$poll_id' 
ORDER BY x.id DESC 

它只是表明答案是它有更多的則0票，這意味着如果我的調查是新的，沒有票就絕不會顯示答案

這樣的問題如何，如果沒有vots更新這個代碼來顯示的答案太

Answer 1

SELECT x.*, z.rates 
FROM poll_answers x 
    LEFT JOIN 
    (SELECT answer_id, COUNT(id) AS `rates` 
    FROM poll_votes 
    GROUP BY answer_id) z 
    ON 
    x.id = z.answer_id 
WHERE x.poll_id = '$poll_id' 
ORDER BY x.id DESC 

Answer 2

嘗試這個

SELECT x . * , z.rates 
FROM poll_answers x 
LEFT JOIN (SELECT answer_id, COUNT(id) AS `rates`  
FROM poll_votes  
GROUP BY answer_id) z  
     ON x.id = z.answer_id 
WHERE x.poll_id = '$poll_id' ORDER BY x.id DESC 

Answer 3

你必須做一個LEFT JOIN，而不僅僅是一個JOIN：

SELECT x.*, z.rates 
FROM poll_answers x 
    LEFT JOIN 
    (SELECT answer_id, COUNT(id) AS `rates` 
    FROM poll_votes 
    GROUP BY answer_id) z 
    ON 
    x.id = z.answer_id 
WHERE x.poll_id = '$poll_id' 
ORDER BY x.id DESC 

JOIN或INNER JOIN用於連接兩個表，並只保留其存在於兩側的行。

LEFT JOIN保留所有行左側表（FROM子句中的表）和右表中添加信息（表你與加盟）時可能

RIGHT JOIN不斷從右邊的所有行表並在可能的情況下從左表中添加信息。

在你的情況，因爲你想要所有的poll_answers你必須使用LEFT JOIN。

請注意，rates將是null而不是0當沒有投票。如果要過濾單個查詢，您應該 - 出於性能原因 - 將poll_answers添加到分組子查詢中，如果您要過濾單個查詢，則可以使用此SELECT x.*, CASE z.rates IS NULL 0 ELSE z.rates替換第一行，如果您想擁有0

Answer 4

，因爲你只想計算一次投票的票數，而不是整個表。

內部查詢應該是這樣的：

(SELECT v.answer_id, COUNT(v.id) AS `rates` 
FROM poll_answers a JOIN poll_votes v 
    ON a.answer_id = v.answer_id 
WHERE a.poll_id = '$poll_id' 
GROUP BY v.answer_id)