How to Recursively Traverse a Tree Structure in MySQL?

Recursing a Tree Structure in MySQL

When working with hierarchical data, it's often necessary to retrieve the descendants of a particular node in the tree structure. In the context of MySQL, where a location can be inside another location, the task becomes more complex as the number of levels increases.

The provided PHP/MySQL query:

$sql = "SELECT id FROM se_locations_services WHERE parent_locationid IN
( SELECT location_id FROM se_locations_parent WHERE parent_id = '$locationid' )";

effectively retrieves the immediate descendants of a given location.

However, to retrieve all the descendants, regardless of depth, a recursive approach is required. MySQL does not natively support recursive queries, but there's an elegant solution outlined in the comprehensive article linked below from mysql.com:

[Managing Hierarchical Data in MySQL](https://www.mysql.com/news-and-events/mysql-perspectives/performance-tuning-json-mysql-for-hierarchical-data/)

This article presents a variety of approaches to handling hierarchical data, including the option of using CTEs (Common Table Expressions) to implement recursion:

WITH RECURSIVE descendants (id, parent_id) AS (
    SELECT id, parent_id FROM locations
    UNION ALL
    SELECT l.id, l.parent_id FROM locations l
    JOIN descendants d ON l.parent_id = d.id
)
SELECT DISTINCT id FROM descendants WHERE parent_id = $locationid;

By employing the UNION ALL operator, the CTE iteratively expands the list of descendants, allowing MySQL to retrieve the full hierarchy from a single query.

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