使用mysql进行递归调用的存储过程

我已尝试使用存储过程与递归调用.我需要根据leg1基于leg1显示相关名称

但它显示错误,例如#1414 – OUT或INOUT参数2用于例行sample.getVolume不是BEFORE触发器中的变量或NEW伪变量

这是我的代码,

DELIMITER $$
CREATE PROCEDURE getVolume( IN param_name VARCHAR(255),OUT result VARCHAR(255 ))
BEGIN 
SELECT val INTO result FROM employee WHERE emp_name = param_name ;
IF result IS NULL THEN
select result;
ELSE     
CALL getVolume(result,'');
END IF;
END $$
DELIMITER ;



SET @@GLOBAL.max_sp_recursion_depth = 255;
SET @@session.max_sp_recursion_depth = 255; 
call getVolume('new',@result);
select @result;

DELIMITER //

DROP TABLE IF EXISTS `employee`//
DROP PROCEDURE IF EXISTS `getVolume`//

CREATE TABLE `employee` (
  `emp_name` VARCHAR(255),`val` VARCHAR(255)
)//

INSERT INTO `employee` (`emp_name`,`val`)
VALUES
('demo','new'),('new','d.new'),('d.new','view'),('view','hello'),('hello',NULL)
//

CREATE PROCEDURE `getVolume`(
  IN `param_name` VARCHAR(255),OUT `result` VARCHAR(255)
)
BEGIN
  DECLARE `next_param_name` VARCHAR(255);
  SELECT `val` INTO `next_param_name`
  FROM `employee`
  WHERE `emp_name` = `param_name`;

  IF `next_param_name` IS NULL THEN
    SET `result` := `param_name`;
  ELSE
    -- CALL `getVolume`(`result`,'');
    CALL `getVolume`(`next_param_name`,`result`);
    -- SET `result` := CONCAT(`param_name`,',IFNULL(`result`,''));
  END IF;
END//

DELIMITER ;

SQL Fiddle demo

（编辑：李大同）

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