這兩個查詢有什麼區別？爲什麼不同的結果

Question

這 - 會產生不正確值total_classes

SELECT IFNULL(count(wc.id_wc),0) AS total_classes 

FROM `all_tag_relations` AS a 
    LEFT JOIN (tags AS t, WebClasses as wc) 
    ON (a.id_tag = t.id_tag AND a.id_tutor = wc.id_author) 
GROUP BY a.id_tutor 

但是這一次 - 使用子查詢提供了正確的價值觀 -

SELECT (SELECT IFNULL(count(wc.id_wc),0) FROM WebClasses as wc WHERE wc.id_author = a.id_tutor) AS total_classes 


FROM `all_tag_relations` AS a 
    LEFT JOIN (tags AS t) ON (a.id_tag = t.id_tag) 
group by a.id_tutor 

解決方案 解決的辦法是加入其他表與實際的Tutors表All_Tag_relations像這樣

...FROM Tutors as td on join All_Tag_Relations as a on td.id_tutor=a.id_tutor LEFT JOIN ... 

Answer 1

您的查詢都很奇怪，並且您不清楚您要做什麼。也許你應該顯示你的表格並要求一個查詢來檢索你想要的數據。

這些查詢之間的區別主要在於第二個給出WebClass的總數，第二個給出的數字ob WebClasses乘以作者/導師數乘以標籤數量，假設它們是不相交的並且標籤，WebClasses和all_tag_relations之間都有1：n的關係。