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tag (
  id
  tag
  raw_tag
)

object_tag (
  tag_id
  object_id
)


exemple :

[tag]
1 | truc | Truc
2 | truc | truc
3 | truc | TRUC
4 | bidule | Bidule
5 | bidule | BIDULE

[object_tag]
1  | 1
2  | 1
3  | 2
4  | 1
5  | 3
6  | 3
7  | 4
8  | 4
9  | 5
10 | 1
11 | 2
12 | 3


SELECT
        tag,
        COUNT (tag_id) as c_tag,
        (select t2.raw_tag
        from tag t2
        join object_tag ot2
                on (t2.id=ot2.tag_id)
        where t2.tag=tag.tag
        group by t2.raw_tag
        order by count(ot2.tag_id) DESC
        limit 1
        ) as raw
FROM    tag
JOIN   object_tag
        ON (tag.id = object_tag.tag_id)
GROUP BY tag
ORDER BY 2 DESC;