inner join and count records recursively - mysql

Question

I have two tables named Topics and DocumentTopics
Topics table has the following structure
di_name | parentTopic
the records in Topics can be either a main or a child topic of a main topic. If a record is a main topic then the value of parentTopic will be null.
DocumentsTopics has the following structure
doc_name | topic_di_name
documentsTopics holds the records of all my documents by name and their topics. I can join both tables using topics->di_name and documentTopics->topic_di_name
SELECT t1.topic_di_name, count(t1.topic_di_name) as count
FROM DocumentsTopics t1
INNER JOIN Topics t2
ON t1.topic_di_name = t2.di_name
GROUP BY t1.topic_di_name;
but what I actually want to is group the topics by main topics only and get a sum count of all the records under the main topic and all its children's topics. These may drill down to more than one level.
here is a schema with sample data
There are 2 main topics but only one has siblings. The total count I am looking for is for the main topics only but they should include the counts of all the sibling topics within.
I am guessing the only way to do that is to run a recursive function but I am unsure about how to do that with a single SQL query. Is it even possible?
Any help would be greatly appreciated

Answer 1

Here's another way to do it using a UNION in a sub query in case this makes more sense to anyone:
SELECT `der`.`parentTopic`,
SUM(IF(`dt`.`topic_di_name` = `der`.`subTopic`,1,0)) AS `counter`
FROM (
SELECT `parentTopic`,
`di_name` AS `subTopic`
FROM `topics`
WHERE `parentTopic` IS NOT NULL
UNION
SELECT `di_name` AS `parentTopic`,
`di_name` AS `subTopic`
FROM `topics`
WHERE `parentTopic` IS NULL) AS `der`
JOIN `documentsTopics` AS `dt` ON `dt`.`topic_di_name` = `der`.`subTopic`
GROUP BY `der`.`parentTopic`
;
Here's this answer in sql fiddle.
Hope this helps!

Answer 2

Why can't you use:
SELECT t3.parent_key, count(t3.parent_key)
FROM DocumentsTopics t1
INNER JOIN Topics t2
ON t1.topic_di_name = t2.di_name
LEFT JOIN (select *, coalesce(parentTopic, di_name) parent_key from Topics) t3
ON t2.di_name = t3.di_name
GROUP BY t3.parent_key

Answer 3

Something like this should work for two levels.
https://www.db-fiddle.com/f/vcjUtUfxsn4yaKnQhmQJ6d/5
SELECT IFNULL(t.parentTopic,t.di_name) AS main_topics,COUNT(*) AS counter
FROM DocumentsTopics d
INNER JOIN Topics t ON d.topic_di_name=t.di_name
GROUP BY 1

Answer 4

I think this will work in mySql 8.0 and give you nested results more than one level deep, but I don't have access to test
with recursive cte as (
SELECT `parentTopic`,
`di_name`
FROM `topics`
WHERE `parentTopic` IS NOT NULL
union all
select
`parentTopic`,
`di_name`
from `topics` t2
inner join cte on t2.`parentTopic` = cte.`di_name`
)
select * from cte