sql查询：如何让父母没有孩子？

Question

我有这个查询来获取我想要的第一个结果 post 早些时候，

SELECT parents.tag_id AS ParentID,
       parents.tag_name AS ParentName,
       COUNT(childs.tag_id) AS TotalChildren
FROM root_tags AS parents
    LEFT OUTER JOIN root_tags AS childs
        ON parents.tag_id = childs.parent_id
WHERE parents.parent_id IS NULL
GROUP BY parents.tag_id, parents.tag_name
ORDER BY parents.tag_id

它返回，

ParentID    ParentName  TotalChildren
3           Tagname-1   2
5           tagname-2   2
6           tagname-3   1
9           tagname-4   1
18          tagname-10  0
24          tagname-13  0
26          tagname-14  0
28          tagname-15  0

但我现在有另一个问题，因为我想返回根本没有孩子的父母，

ParentID    ParentName  TotalChildren
18          tagname-10  0
24          tagname-13  0
26          tagname-14  0
28          tagname-15  0

所以我尝试对此进行查询，

SELECT 
    parents.tag_id AS ParentID,
    parents.tag_name AS ParentName,
    COUNT(childs.tag_id) AS TotalChildren

FROM root_tags AS parents
    LEFT OUTER JOIN root_tags AS childs
    ON parents.tag_id = childs.parent_id

WHERE parents.parent_id IS NULL
AND COUNT(childs.tag_id) = '0'
GROUP BY parents.tag_id, parents.tag_name
ORDER BY parents.tag_id

但它返回一个错误<代码>#1111 -群组功能使用无效

如何修复？

Answer 1

只需在原始查询中添加一个 HAVING 子句即可：

SELECT parents.tag_id AS ParentID,
       parents.tag_name AS ParentName,
       COUNT(childs.tag_id) AS TotalChildren
FROM root_tags AS parents
    LEFT OUTER JOIN root_tags AS childs
        ON parents.tag_id = childs.parent_id
WHERE parents.parent_id IS NULL
GROUP BY parents.tag_id, parents.tag_name
HAVING COUNT(childs.tag_id) = 0
ORDER BY parents.tag_id

您的问题是您尝试将 WHERE 子句与聚合和函数 (COUNT) 一起使用。但您应该在聚合之前使用 WHERE 过滤数据。 HAVING 用于过滤聚合后的结果。

Answer 2

将此部分更改

AND COUNT(childs.tag_id) = '0'
GROUP BY parents.tag_id, parents.tag_name

为

GROUP BY parents.tag_id, parents.tag_name
HAVING COUNT(childs.tag_id) = 0

Answer 3

使用 HAVING 代替 WHERE，并在 GROUP BY

http://blog.cnizz.com/ 之后tag/mysql-error-1111/

这可能很有用。