这两个查询有什么区别?为什么结果不同?

这个 -给出的值不正确 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

但是这个 -使用子查询提供正确的值-

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

解决方案 解决方案是将其他表与具有如下所有标记关系的实际导师表连接起来

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