php - sql 查询 : how to make the parents without children?

Question

我有这个查询来从这个 post 得到我想要的第一个结果早些时候，

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

哪个返回，

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 child 的 parent ，

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

所以我试图对此进行查询，

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

但它返回错误 #1111 - 组函数的使用无效

我该如何解决？

Answer 1

只需在原始查询中添加一个 HAVING 子句:

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

您的问题是您试图将 WHERE 子句与聚合和函数 (COUNT) 结合使用。但是您应该使用 WHERE 来过滤数据 before 聚合。 HAVING 用于在聚合 之后过滤结果。