如何統計多個表中的多個事物

Question

我總是看到COUNT（）與*作爲參數使用，GROUP BY決定什麼是要計算的，但我相信你可以計算不同的東西必須做子查詢，分組，然後加入（這就是我現在正在做的）。

讓我們建立一個例子出來，這樣一個簡化版本：

Table Question 
idQuestion | title | idUser 

Table Answer 
idAnswer | idQUestion | text | idUser 

Table Vote 
idVote | idAnswer | type | idUser 

Table Comment 
idComment | idAnswer | text | idUser 

要知道所有的答案多少評論和投票有我做：

SELECT idAnswer, votes, comments FROM 
    (SELECT idAnswer, count(*) AS votes FROM Answer A 
     JOIN Vote B ON A.idAnswer=B.idAnswer 
    GROUP BY idAnswer) AS X 
JOIN 
    (SELECT idAnswer, count(*) AS comments FROM Answer A 
     JOIN Comment B ON A.idAnswer=B.idAnswer 
    GROUP BY idAnswer) AS Y 
ON X.idAnswer=Y.idAnswer 

可以這樣沒有完成子選擇？什麼是通過避免它們來實現的？

如何計算同一查詢中的負面投票和正面投票？

Answer 1

SELECT idAnswer, COUNT(DISTINCT VOTE.idVote), COUNT(DISTINCT comments) 
FROM Answer 
LEFT JOIN COMMENT ON Answer.idAnswer = Comment.idAnswer 
LEFT JOIN VOTE ON Answer.idAnswer = Vote.idAnswer 
GROUP BY idAnswer 

至於你的問題的第二部分，感謝Andriy M！

SELECT 
    Answer.idAnswer, 
    COUNT(DISTINCT CASE WHEN VOTE.type = 'up' THEN VOTE.idVote END) AS upvotes, 
    COUNT(DISTINCT CASE WHEN VOTE.type = 'down' THEN VOTE.idVote END) AS downvotes, 
    COUNT(DISTINCT comments) as comments 
FROM Answer 
LEFT JOIN COMMENT ON Answer.idAnswer = Comment.idAnswer 
LEFT JOIN VOTE ON Answer.idAnswer = Vote.idAnswer 
GROUP BY Answer.idAnswer