SQL - Not existing rows as existing

I have 3 tables. One with a list of users, one with items and one with relation between the users and the items. User can have an item (stat = 0/1) or can have a duplicate (dup = 0/1). If you click on the item's or the duplicate's button, it runs a query that adds or updates the row.

It looks like this:

| user | item_id | stat | dup |
| ---- | ------- | ---- | --- |
| 17   |   1     |  1   |  1  |
| 5    |   2     |  1   |  0  |
| 8    |   1     |  0   |  1  |
| 9    |   4     |  1   |  0  |

I compare these data who have an item what the other does not have and vica versa with this query:

SELECT t2.user_id,
       GROUP_CONCAT(CASE WHEN t1.dup=1 AND t2.stat=0 THEN item_id END) `My Item List`,
       GROUP_CONCAT(CASE WHEN t2.dup=1 AND t1.stat=0 THEN item_id END) `Item List`
FROM table3 t1
LEFT JOIN table3 t2 USING (item_id)
WHERE t1.user_id = @current_user
  AND t2.user_id <> @current_user
GROUP BY t2.user_id

My problem is I can only compare the rows if the user already clicked the buttons at least once, so the row generated, but I need the stat=0 and dup=0 rows as well even if they are not existing.

How can I integrate this to the query above?

My tables:

Table1

CREATE TABLE table1(
    id NOT NULL AUTO_INCREMENT,
    user_name varchar(255),
);

Table 2

CREATE TABLE table2(
    id NOT NULL AUTO_INCREMENT,
    item_no varchar(255),
    item_name varchar(255),
    item_group varchar(255)
);

Table 3

CREATE TABLE table3 (
  id int NOT NULL AUTO_INCREMENT,
  user_id int NOT NULL,
  item_id int NOT NULL,
  stat tinyint NOT NULL,
  dup tinyint NOT NULL,
  PRIMARY KEY (id),
  FOREIGN KEY (user_id) REFERENCES table1(id),
  FOREIGN KEY (item_id) REFERENCES table2(id)
);

Thank you!

First of all I would highly advise you to change the names of your Tables to something more meaningful so you (and we) don't get confused.

--We are going to group all of the Item Ids in one variable
DECLARE @All_Item_ids VARCHAR(MAX)
SET @All_Item_ids =SELECT STUFF((SELECT ', ' + t2.id 
          FROM table2 t2
          FOR XML PATH('')), 1, 1, '') 

--We are going to group all of the Item Ids that belong to a user in one variable
DECLARE @User_Item_ids VARCHAR(MAX)
SET @User_Item_ids =SELECT STUFF((SELECT ', ' + t2.id 
          FROM table2 t2 
          WHERE t2.id IN (SELECT item_id FROM table3 WHERE user_id = @current_user)
          FOR XML PATH('')), 1, 1, '')

--We are showing the result
SELECT @current_user,@User_Item_ids 'My Item List' ,@All_Item_ids 'Item List'