sql查询:如何让父母没有孩子？

Question

我使用这个查询来获得我想要的第一个结果，来自前面的post，

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，因为我想把那些根本没有孩子的父母还回去，

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

所以我试着对此进行查询，

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

但是它返回一个错误#1111 - Invalid use of group function

我怎么才能修复它？

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

您的问题是您试图在聚合和函数(COUNT)中使用WHERE子句。但是在聚合之前，您应该使用WHERE来过滤数据。HAVING用于对聚合后的结果进行过滤。

Answer 2

更改此部件

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

至

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

Answer 3

使用HAVING而不是WHERE，以及在GROUP BY之后

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

这可能会很有用。