How to Recursively Retrieve Hierarchical Child Folders in MySQL with Varying Levels?

Recursive MySQL Query to Retrieve Hierarchical Child Folders with Varying Levels

When managing hierarchical data structures, such as nested folders or tree-like relationships, finding all the children and their potential descendants can be challenging. This article presents a MySQL query that effectively addresses this need by recursively traversing a hierarchical 'Folder' table.

The 'Folder' table used in this example has three columns: 'idFolder', 'FolderName', and 'idFolderParent', which represents the parent-child relationship. The goal is to fetch all child folders, including subchild folders, for a given 'idFolder' in a single query.

One of the challenges faced by previous solutions was handling cases where parent folders were not saved in a specific order in the database. To overcome this, we propose a query that relies on the GROUP_CONCAT() function and a custom variable.

The query:

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;

This query works as follows:

1. It initializes a custom variable @pv with the 'idFolder' of the target folder (in this case, 5).
2. The inner query recursively searches for all child 'idFolder' values that have @pv as their parent.
3. The results are grouped and concatenated, separated by commas, to form a string containing the 'idFolder' values of all child and subchild folders.

This approach ensures that all child folders and subchild folders are retrieved regardless of their position in the hierarchical structure, providing a comprehensive list for the given 'idFolder' value.

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