Get all parents of multiple objects in same table - mysql

Question

for an mass edit function I need to load the parents of multiple objects.
Doing this with single querys would kill the db. I'm using MySQL 5.7 My table is build like this:
CREATE TABLE `testtable` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`content` text,
`parentid` bigint(20) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
INSERT INTO testtable(`id`, `content`, `parentid`)
VALUES(1, 'parentA1', 0),(5, 'parentA2', 1),(3, 'childA', 2),
(4, 'parentB', 0),(5, 'childB', 4);
For single object querys I use this statement to get all parents:
SELECT t.id, t.content, #pid := t.parentid AS parentid
FROM (SELECT * FROM Table1 order by id DESC) t
JOIN (SELECT #pid := 3) tmp
WHERE t.id = #pid
But I have absolutly no clue how this could work for multiple object at once without using union.
My whised ouput should look like this:
id | content | parentid | searchingChildID
1 | parentA1| 0 | 3
2 | parentA2| 1 | 3
3 | childA | 2 | 3
4 | parentB | 0 | 5
5 | childB | 4 | 5
Thanks in advance!

Answer 1

If you are running MySQL 8.0, this is a typical use case for a recursive query. Say you want all parents of objects 3 and 4, you can do:
with recursive cte as (
select t.id as originalid, t.* from table1 where id in (3, 4)
union all
select c.originalid, t.*
from cte c
inner join table1 t on t.id = c.parentid
)
select * from cte

Answer 2

for an mass edit function I need to load the parents of multiple objects.
For this operation, I would expect:
id | content | parentid | ultimateparent
1 | parentA1| 0 | 0
2 | parentA2| 1 | 0
3 | childA | 2 | 0
4 | parentB | 0 | 0
5 | childB | 4 | 0
Because 0 is the ultimate parent of all the rows. If you know the maximum depth, you can use left joins in pre-8 versions of MySQL:
select t1.*, coalesce(t3.parentid, t2.parentid, t1.parentid) as ultimateparent
from testtable t1 left join
testtable t2
on t2.id = t1.parentid left join
testtable t3
on t3.id = t2.parentid
You can trivially extend this with more left joins to handle deeper levels of parentage.
If 0 really means "no parent" (which is an odd choice when NULL is available) then you would seem to want:
id | content | parentid | ultimateparent
1 | parentA1| 0 | 1
2 | parentA2| 1 | 1
3 | childA | 2 | 1
4 | parentB | 0 | 4
5 | childB | 4 | 4
Then you can actually just tweak the above query to:
select t1.*, coalesce(t3.id, t2.id, t1.id) as ultimateparent
from testtable t1 left join
testtable t2
on t2.id = t1.parentid left join
testtable t3
on t3.id = t2.parentid;
Here is a db<>fiddle.
You can get the maximum child for each parent as well (although that seems quite arbitrary). In pre-8.0 versions of MySQL, variables are the simplest approach:
select t1.*,
(#max_child := if(#up = ultimateparent, #max_child,
if(#up := ultimateparent, id, id)
)
) as max_childid
from (select t1.*, coalesce(t3.id, t2.id, t1.id) as ultimateparent
from testtable t1 left join
testtable t2
on t2.id = t1.parentid left join
testtable t3
on t3.id = t2.parentid
order by ultimateparent, id desc
) t1 cross join
(select #up := -1, #max_child := -1) params;