三表的連接和計數顯示我有3個表錯誤的結果

Question

，我需要從目錄中的所有信息，請加入參數表，並加入到意見表，再算上通過目錄發佈的評論，我的SQL查詢：

SELECT 
    catalog.catalog_id, 
    catalog.slug, 
    catalog.title, 
    catalog.city, 
    catalog.street, 
    catalog.image COUNT(ratings.rate) AS votes, 
    COUNT(comments.catalog_id) AS total_comments, 
    ROUND(SUM(ratings.rate)/COUNT(ratings.rate)) AS average 
FROM 
    catalog 
LEFT JOIN ratings ON ratings.object_id = catalog.catalog_id 
LEFT JOIN comments ON comments.catalog_id = catalog.catalog_id 
GROUP BY 
    catalog.catalog_id 
ORDER BY 
    average, 
    votes DESC 

一切都顯示很好只有total_comments壞數字6，但在評論表中只有2行，所以它的壞結果。我認爲這是分組的問題。我試過添加GROUP BY catalog.catalog_id, comments.catalog_id，但沒有幫助。 我的表：

Answer 1

的問題是，你有多個評級和評論，所以你得到的每一個職位的笛卡爾乘積。

正確的解決方案是在加入前預先聚合數據。

SELECT c.*, r.votes, c.total_comments, 
     ROUND(sumrate/votes) AS average 
FROM catalog c LEFT JOIN 
    (SELECT r.object_id, COUNT(*) as votes, SUM(r.rate) as sumrate 
     FROM ratings r 
     GROUP BY r.object_id 
    ) r 
    ON r.object_id = c.catalog_id LEFT JOIN 
    (SELECT c.catalog_id, COUNT(*) as total_comments 
     FROM comments c 
     GROUP BY c.catalog_id 
    ) c 
    ON c.catalog_id = c.catalog_id 
GROUP BY c.catalog_id 
ORDER BY average, votes DESC;