sql query:bagaimana cara membuat tag tanpa anak menjadi orang tua?

Anda hampir sampai.. hanya perlu membuat gabungan menjadi bagian luar:

DIEDIT:

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
WHERE b.TotalChildren is not null
ORDER BY ParentID