MySQL use DISTINCT in GROUP_CONTACT on a different column than the output one

Question

I'm trying to use GROUP_CONCAT with DISTINCT, but by executing the distinction on a different column. For example, I'm doing this:

GROUP_CONCAT(DISTINCT tags.tag SEPARATOR ' ') as tags"

But a tag could be the same characters with a different id, so I would need to do something like this:

GROUP_CONCAT(DISTINCT(tags.tag_id) tags.tag SEPARATOR ' ') as tags"

However I can't find anything about this, any ideas?

SIMPLIFIED table data examples (model and SQL request are a lot more complex with multiple joins and tables):
items table:
item_id , title , ...
1, Smith, ...
2, Bob, ...
...

tags table:
tag_id , tag , item_id , ...
1, Montreal, 1, ...
2, Toronto, 1, ...
3, Toronto, 1, ...
4, New-York, 2, ...
...

The MySQL query:

...
SELECT items.item_id, items.title,
GROUP_CONCAT(DISTINCT tags.tag_id SEPARATOR ',') as tag_ids",
GROUP_CONCAT(DISTINCT tags.tag SEPARATOR ' ') as tags"
LEFT JOIN tags ON items.item_id = tags.item_id
GROUP BY items.item_id
..

Expected result:
item1.title = "Smith"
item1.tag_ids = "1 2 3"
item1.tags = "Montreal Toronto Toronto"
...

Actual result:
item1.title = "Smith"
item1.tag_ids = "1 2 3"
item1.tags = "Montreal Toronto" <-- MISMATCH IN TAGS COUNT BECAUSE OF DISTINCT
...

NOTE: I would like to avoid sub-queries like

SELECT WHERE tags.item_id IN(SELECT ...)

Answer 1

I'm not 100% sure what you want but if I'm right, you want distinct tag_ids and then corresponding tag names, so 'montreal toronto toronto' would be fine for you:

SELECT item_id,
       GROUP_CONCAT(tag_id SEPARATOR ' ') AS tag_ids,
       GROUP_CONCAT(tag SEPARATOR ' ') AS tags
FROM
    (SELECT items.item_id AS item_id,
            tags.tag_id AS tag_id,
            tags.tag AS tag
     FROM items
     JOIN tags ON tags.item_id = items.item_id
     GROUP BY 1,
              2) AS distinct_groups
GROUP BY item_id