MySQL recursively query all child nodes, tree structure query

CREATE FUNCTION fn_get_child(rootId INT,deep INT) RETURNS varchar(4000) CHARSET utf8 #rootId is the node you want to query #deep is the level you want to query
BEGIN
#declare two temporary variables
DECLARE temp VARCHAR(4000);
DECLARE temp_child VARCHAR(4000);
DECLARE n INT DEFAULT 2;
SET temp = '$';
SET temp_child = CAST(rootId AS CHAR);#Force rootId to character
SET n = deep;
WHILE temp_child is not null DO
SET temp = CONCAT(temp,',',temp_child);#The loop connects all nodes into a string.
SELECT GROUP_CONCAT(id) INTO temp_child FROM address where FIND_IN_SET(parent_id,temp_child)>0;
set n = n-1;
END WHILE;
RETURN temp;

END