DELIMITER //
CREATE FUNCTION `getChildLst`(rootId INT)
 
RETURNS varchar(1000) READS SQL DATA
 
BEGIN
 DECLARE sTemp VARCHAR(1000);
 
 DECLARE sTempChd VARCHAR(1000);
 
 SET sTemp = '$';
 
 SET sTempChd =cast(rootId as CHAR);
 
 WHILE sTempChd is not null DO
 
 SET sTemp = concat(sTemp,',',sTempChd);
 
 SELECT group_concat(id) INTO sTempChd FROM document_file_name where FIND_IN_SET(pId,sTempChd)>0;
 
 END WHILE;
 
 RETURN sTemp;
 
END //

select * from document_file_name where find_in_set(id, getChildLst(1));