MySQL – Recursively list all parents and ancestors of all items in table

I have a table with a parent/child hierarchy that supports multiple (theoretically infinite) levels of nesting:

|------|-------------------|-------------|
|  id  |       title       |  parent_id  |
|------|-------------------|-------------|
|  1   |    Dashboard      |      0      |
|  2   |    Content        |      0      |
|  3   |    Modules        |      0      |
|  17  |    User Modules   |      3      |
|  31  |    Categories     |      17     |
|  ... |                   |             |
|------|-------------------|-------------|

I am trying to build a query that produces a concatenated list of every item’s parent items up until the highest parent in the tree:

|------|----------------------|
|  id  | concatenatedParents  |
|------|----------------------|
|  1   |  0                   |
|  2   |  0                   |
|  3   |  0                   |
|  17  |  3,0                 |
|  31  |  17,3,0              |
|  ... |                      |
|------|----------------------|

Based on a number of other answers here I have constructed the following MySQL query:

SELECT parentsTable._id, GROUP_CONCAT(parentsTable.parent_id SEPARATOR ',') as concatenatedParents FROM (
    SELECT
        @r AS _id,
        (SELECT @r := parent_id FROM menu WHERE id = _id) AS parent_id,
        @l := @l + 1 AS lvl
    FROM
        (SELECT @r := 31, @l := 0) vars,
        menu m
    WHERE @r <> 0
) as parentsTable

See Fiddle here: http://sqlfiddle.com/#!9/48d276f/902/0

But this query only works for one given child id (31 in this example). I did not succeed to expand this query for the whole table, is there some way to reset the counter variables at every next row in the table?

I have seen many answers that suggest using a fixed number of joins, but a solution that accepts a variable number of levels would be much more preferable.

In MySQL 8 this is possible thanks to recursive queries (thank you @GMB), but since we are still running on MySQL 5.7 I am interested if a solution exists for older versions as well.

Answer

If you are running MySQL 8.0, this is best solved with a recursive query:

with recursive cte as (
    select id, parent_id, 1 lvl from mytable
    union all
    select c.id, t.parent_id, lvl + 1
    from cte c
    inner join mytable t on t.id = c.parent_id
)
select id, group_concat(parent_id order by lvl) all_parents
from cte
group by id

Demo on DB Fiddle:

id | all_parents
-: | :----------
 1 | 0          
 2 | 0          
 3 | 0          
17 | 3,0        
31 | 17,3,0