Question

I have this tag table,

tag_id  tag_name    parent_id   cat_id
3       Tagname-1   NULL        NULL    
5       tagname-2   NULL        NULL    
6       tagname-3   NULL        NULL
9       tagname-4   NULL        NULL
11      tagname-5   3           NULL
13      tagname-6   3           NULL
15      tagname-8   5           NULL
17      tagname-9   5           NULL
18      tagname-10  NULL        NULL
20      tagname-11  6           NULL
22      tagname-12  9           NULL
24      tagname-13  NULL        NULL
26      tagname-14  NULL        NULL
28      tagname-15  NULL        NULL

I want to return a result like this,

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

So here the query I came out with so far,

SELECT 
a.tag_id as ParentID,
a.tag_name as ParentName,
b.TotalChildren

FROM root_tags a INNER JOIN
(
    SELECT parent_id, COUNT(1) as TotalChildren
    FROM root_tags
    WHERE parent_id <> tag_id
    GROUP BY parent_id
) b 

ON a.tag_id = b.parent_id
ORDER BY ParentID

But, unfortunately, it only return the result like this,

ParentID    ParentName  TotalChildren
3           Tagname-1   2
5           tagname-2   2
6           tagname-3   1
9           tagname-4   1

Which means it is missing the parents without any children.

How can I make the tags without children to become parents too? or in other words, how to make the tags without parents to become parents themselves?

EDIT:

SELECT 
a.tag_id as ParentID,
a.tag_name as ParentName,
b.TotalChildren

FROM root_tags a LEFT OUTER JOIN
(
    SELECT parent_id, COUNT(1) as TotalChildren
    FROM root_tags
    WHERE parent_id <> tag_id
    GROUP BY parent_id
) b 

ON a.tag_id = b.parent_id
ORDER BY ParentID

the answer above returns,

ParentID    ParentName  TotalChildren
3           Tagname-1   2
5           tagname-2   2
6           tagname-3   1
9           tagname-4   1
11          tagname-5   NULL
13          tagname-6   NULL
15          tagname-8   NULL
17          tagname-9   NULL
18          tagname-10  NULL
20          tagname-11  NULL
22          tagname-12  NULL
24          tagname-13  NULL
26          tagname-14  NULL
28          tagname-15  NULL

which is incorrect as it returns all the children.

Answer 1

Prefer a Join against a Subquery:

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