MySQL查詢，嘗試收集計數

Question

我正在嘗試編寫一個查詢，以獲取數據庫中每個recordlabel的藝術家和樂隊的數量。 出於某種原因，當我運行以下查詢時，它沒有列出數據庫中分配給樂隊或藝術家的每個唱片公司標簽。

 SELECT  r.labelName, 
        COUNT(a.recordLabelID) +COUNT(b.recordLabelID) AS count 
        FROM recordLabel r JOIN artist a ON r.recordLabelID = a.recordLabelID
        JOIN band b ON r.recordLabelID = b.recordLabelID
        GROUP BY r.recordlabelID ORDER BY count DESC

Answer 1

嘗試使用left join ：

SELECT r.labelName, COUNT(a.recordLabelID) + COUNT(b.recordLabelID) AS count
FROM recordLabel r JOIN
     artist a
     ON r.recordLabelID = a.recordLabelID JOIN
     band b
     ON r.recordLabelID = b.recordLabelID
GROUP BY r.recordlabelID
ORDER BY count DESC;

但是，這將不起作用，因為計數將返回相同的值。 一個簡單的解決方法是使用COUNT(DISTINCT) ：

SELECT r.labelName,
       COUNT(DISTINCT a.recordLabelID) + COUNT(DISTINCT b.recordLabelID) AS count
FROM recordLabel r LEFT JOIN
     artist a
     ON r.recordLabelID = a.recordLabelID LEFT JOIN
     band b
     ON r.recordLabelID = b.recordLabelID
GROUP BY r.recordlabelID
ORDER BY count DESC;

您需要使用LEFT JOIN ，以防某些標簽上有歌手但沒有樂隊，反之亦然。

更具擴展性的解決方案是在進行聯接之前進行聚合。

Answer 2

使用LEFT JOIN ：

FROM recordLabel r 
LEFT JOIN artist a ON r.recordLabelID = a.recordLabelID
LEFT JOIN band b ON r.recordLabelID = b.recordLabelID