Question

SELECT categories.*, COUNT(categoryID) AS kritCount
FROM categories AS categories
LEFT JOIN krits ON categories.id = categoryID
WHERE (krits.approved = '1')
GROUP BY categories.id

所以这很有效，只是它没有返回类别中有0个krits的类别。

如果我删除WHERE语句但是我需要WHERE只选择字段批准的krits = 1

Answer 1

每次从where子句中引用左连接表中的列时（除了测试NULL值），都会强制该连接的行为类似于内连接。相反，将测试移出where子句并使其成为连接条件的一部分。

SELECT categories. * , COUNT(categoryID) AS kritCount 
    FROM categories AS categories
        LEFT JOIN krits 
            ON categories.id = categoryID
                AND krits.approved = '1'
    GROUP BY categories.id

Answer 2

试试这个：

SELECT categories. * , COUNT(categoryID) AS kritCount FROM categories AS categories
LEFT JOIN krits ON categories.id = categoryID
WHERE (krits.approved = '1' OR krits.approved IS NULL)
GROUP BY categories.id

Answer 3

通过阅读查询，您看起来想要调用类别，并且对于每个类别，您需要针对相应类别计算Krits（已批准），如果没有，则仍需要类别，但显示数为0 ......

试试这个

select
      cat.*,
      COALESCE( kc.KritCount, 0 ) as KritCount
   from 
      Categories cat
         left join ( select k.CategoryID, 
                            count(*) KritCount
                        from 
                           Krits k
                        where
                           k.approved = '1'
                        group by 
                           k.CategoryID ) kc
            on cat.id = kc.CategoryID

mySQL JOIN不会返回0计数的结果

3 个答案: