[SQL] Retrieves results in a hierarchy starting from a specific register (recursive parent_id)

hierarchical_parent_id_recursive_query.sql

/**
 * Retrieves a customer group hierarchy.
 * Change the @root_id value to change the hierarchy root.
 */
SET @root_id = 1;

SELECT  *
FROM (
  SELECT * FROM tbl
    ORDER BY parent_id, id) hiearchy_sorted,
    (SELECT @pv := @root_id) root_id
WHERE 
	find_in_set(parent_id, @pv)
	AND length(@pv := concat(@pv, ',', id));