How to Retrieve All Descendants in a Hierarchical Structure Using MySQL?

Retrieving Hierarchical Child Data in MySQL: Multilevel Recursive Selection

In the realm of database queries, it often becomes necessary to extract not just immediate child data but also multilevel descendent information. This is particularly relevant in hierarchical structures, such as organizational charts or nested directory systems.

Consider a database table named Folder that models a file directory. Each folder has a unique ID, a name, and an optional parent folder ID. Our ultimate goal is to devise a query that, given a specific parent folder ID, can retrieve all its child folders, subchild folders, and so on, effectively providing a comprehensive hierarchical view.

Query Analysis

The provided query leverages MySQL's GROUP_CONCAT() function to aggregate the hierarchical information. It employs a nested SELECT statement to iteratively retrieve all child folders by concatenating their IDs in a comma-separated list. This list is then stored in a variable lv and used as a filter to progressively identify additional levels of children.

The outermost SELECT statement simply groups the concatenated list of child IDs for each iteration and presents them as a comma-separated string. By utilizing the FIND_IN_SET() function, the query ensures that each child is searched within the set of previously identified child IDs.

Execution

Consider the example dataset:

idFolder	FolderName	idFolderParent
1	ADoc	NULL
2	ADoc1	7
3	ADoc2	2
4	ADoc3	3
5	ADoc4	NULL
6	ADoc5	5
7	ADoc6	5

To retrieve all child folders and their descendants for the parent folder ID 5, execute the following query:

<code class="sql">SELECT GROUP_CONCAT(lv SEPARATOR ',') FROM (
SELECT @pv:=(SELECT GROUP_CONCAT(idFolder SEPARATOR ',') FROM Folder 
WHERE FIND_IN_SET(idFolderParent, @pv)) AS lv FROM Folder 
JOIN
(SELECT @pv:=5) tmp
) a;</code>

The result will be a comma-separated list of all child and subchild IDs:

6,7,2,3,4

Conclusion

The provided query offers a robust solution to retrieving hierarchical child data in MySQL, providing a comprehensive view of the folder structure and its nested relationships. It effectively traverses the hierarchy without requiring any modifications to the database structure or relying on predefined ordering of parent-child relationships. This solution proves particularly valuable in managing complex hierarchical data structures.

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