Mysql recursive query to get parent category - mysql

Question

I have 3 tables:
users
user_groups
groups
A user can be in multiple (sub)groups. They are stored in the user_groups table like this
+--------------+--------------+---------------+
| id | user_id | group_id |
+--------------+--------------+---------------+
| 1 | 1 | 23 |
+--------------+--------------+---------------+
| 2 | 2 | 24 |
-----------------------------------------------
Now in my groups table, the top categories are parent_id = 0
+--------------+--------------+---------------+
| id | parent_id | name |
+--------------+--------------+---------------+
| 1 | 2 | Group 1.1 |
+--------------+--------------+---------------+
| 2 | 0 | Group 1 |
+--------------+--------------+---------------+
| 3 | 2 | Group 1.2 |
+--------------+--------------+---------------+
| 4 | 3 | Group 1.2.1 |
+--------------+--------------+---------------+
| 5 | 2 | Group 1.3 |
+--------------+--------------+---------------+
Now I want to build a query which gives me all the parent groups for all users. I did some research about recursive queries and I found this particular post:
How to create a MySQL hierarchical recursive query
But I have no idea how I should approach this when I join the tables.
This is what I got so far:
SELECT
`users`.`id`,
`users`.`first_name`,
`users`.`last_name`,
`users`.`email`,
`users`.`language`,
`groups`.`name`,
`groups`.`parent_id`
FROM `users`
LEFT JOIN `user_groups`
ON `user_groups`.`user_id` = `users`.`id`
LEFT JOIN `groups`
ON `groups`.`id` = `user_groups`.`group_id`
WHERE
`users`.`created`
BETWEEN
DATE_SUB(NOW(), INTERVAL 365 DAY) AND NOW()
But this query just gets me the name and the id of the subgroup. What I want is the top level group.
Thanks for the help!

Answer 1

The typical solution is to create a stored function that returns the top-level group for any given group by tracing the parentage up until it finds a row with parent_id = 0.
Then you can apply that function to each group that the user is a member of, and select the distinct set of top level groups.
Something like this should work for you:
delimiter $$
drop function if exists get_top_level_group_id $$
create function get_top_level_group_id (p_group_id int) returns int
begin
declare v_return_val int;
declare v_group_id int;
declare v_parent_id int;
declare continue handler for not found
begin
return -1;
end;
set v_group_id = p_group_id;
set v_parent_id = p_group_id;
while v_parent_id != 0
do
set v_group_id = v_parent_id;
select `parent_id`
into v_parent_id
from `groups`
where id = v_group_id;
end while;
return v_group_id;
end $$
delimiter ;
Then you can update your query like this to get those users and their distinct top-level groups:
SELECT DISTINCT
`users`.`id`,
`users`.`first_name`,
`users`.`last_name`,
`users`.`email`,
`users`.`language`,
get_top_level_group_id(`user_groups`.`group_id`) as top_level_group_id
FROM `users`
LEFT JOIN `user_groups`
ON `user_groups`.`user_id` = `users`.`id`
WHERE
`users`.`created`
BETWEEN
DATE_SUB(NOW(), INTERVAL 365 DAY) AND NOW()