How to get items from sys_categories - typo3

Question

what is the best way to get the items from categories.
I have a project with approx 40 categories and in total 3000 items and 4 level.
I found a way from TYPO3 self by collections but i must add them to the category manual?
I saw that the TYPO3 blog extension takes another way by an extra column "posts" and a objectstorage in the model categories. So not necessary to add the items / posts.
Somehow the second way like the blog does, sounds nicer but is this performant, when i have a list of 3 categories with each 4 to 5 subcategories and each approx 100 to 250 items?
Thank you

Answer 1

you could build a query and get all related items yourself from the DB.
// get QueryBuilder for table 'sys_category_record_mm'
$queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable('sys_category_record_mm');
// query the table
$result = $queryBuilder
->select('uid_foreign,tablenames')
->from('sys_category_record_mm')
->where($queryBuilder->expr()->eq('uid_local', $uidOfYourCategory))
->execute()
->fetchAll();
Now, $result is an array with all childs of the category wit the id $uidOfYourCategory. This array you can use to get the items from their repos.
If you only want items from tt_content, or maybe a table named tx_yourtable, you can add an andWhere to the query lik this:
$result = $queryBuilder
->select('uid_foreign,tablenames')
->from('sys_category_record_mm')
->where($queryBuilder->expr()->eq('uid_local', $uidOfYourCategory))
->andWhere($queryBuilder->expr()->eq('tablenames', 'tx_yourtable'))
->execute()
->fetchAll();