MySQL数据库创建函数报错提示无法在函数中返回结果集

问题描述：执行创建函数代码时报错，我的MySQL版本是5.5.54

贴源码，不要贴图片

如下：

DELIMITER $$

CREATE FUNCTION myf2(empName VARCHAR(20)) RETURNS DOUBLE
DETERMINISTIC
BEGIN
DECLARE sal double;
set sal=0;
SELECT salary INTO sal
FROM employees
WHERE empName = last_name;
RETURN sal;
END$$

DELIMITER;

DELIMITER $
CREATE FUNCTION myf2(empName VARCHAR(20)) RETURNS DOUBLE 
BEGIN
SELECT @sal :=0;
SELECT salary INTO @sal
FROM employees
WHERE empName = last_name;
RETURN @sal;

END $