SQL-没有评论时，order by破坏了我的查询

Question

我有下面列出的相当长的SQL查询。如您所见，它是通过AvgRating和NumReviews进行排序的，它们均依赖于来自reviews表的数据。不幸的是，即使没有评论，我也需要查看结果中的行，当前，如果届时没有要订购的评论，则该行不会显示在结果中。感谢所有帮助。

SELECT travisor_tradesperson.name, travisor_tradesperson.id, travisor_catagory.catname, 
                    travisor_company.cname, travisor_company.description, travisor_company.city, travisor_company.address, travisor_company.postcode, travisor_company.phone, 
                    ROUND(AVG(travisor_review.rating)) as RoundAvgRating, AVG(travisor_review.rating) as AvgRating, COUNT(travisor_review.rating) as NumReviews
            FROM `travisor_tradesperson`
            INNER JOIN travisor_company 
            ON travisor_tradesperson.company = travisor_company.id
            INNER JOIN travisor_catagory 
            ON travisor_tradesperson.catagory = travisor_catagory.id        
            INNER JOIN travisor_review 
            ON travisor_review.tradesperson = travisor_tradesperson.id 
            WHERE travisor_catagory.catname = '$catagory'
            AND travisor_company.city = '$city'
            GROUP BY travisor_tradesperson.name, travisor_catagory.catname, travisor_company.cname, 
                travisor_company.description
            ORDER BY AvgRating DESC, NumReviews DESC

Answer 1

左联接travisor_review而不是内部联接。内部联接将仅查找两个表中都存在的记录。如果您对该商人记录没有任何评论，它将从结果集中删除。

如果左联接不能与联接谓词匹配，则它将返回NULL。在这种情况下，商人将返回但带有NULL。如果需要，将NULL转换为0，这应该可以修复您的AVG。