如何在oracle中启动过程?

我有一个SELECT语句:

     SELECT emp.emp_name, pos_emp.POSITION_NAME
     from EMPLOYEE emp
     join POSITION_EMPLOYEE pos_emp
     on emp.POSITION_EMPLOYEE_POSITION_ID=pos_emp.POSITION_ID
     where emp.EMP_NAME='&employee_name';

当我从键盘输入员工姓名时,它会返回他的姓名(来自employee表)和职位(来自position_employee表)。但我想用存储过程来实现这一点:

    create or replace PROCEDURE emp_pos(   EMPLOYEE_NAME IN EMPLOYEE.EMP_NAME%TYPE,
                                       POSITION_NAME OUT POSITION_EMPLOYEE.POSITION_NAME%TYPE )
AS
BEGIN
    SELECT pos_emp.POSITION_NAME
    INTO
        POSITION_NAME
    FROM EMPLOYEE emp
        JOIN
            POSITION_EMPLOYEE pos_emp
            ON
                emp.POSITION_EMPLOYEE_POSITION_ID 
                = pos_emp.POSITION_ID
    WHERE emp.EMP_NAME 
          = EMPLOYEE_NAME
    ;

END;

我试图用begin开始这个过程:

begin emp_pos('&employee_name');
end;

编译器给了我错误:调用“EMP\u POS”时参数的数量或类型错误。我哪里错了?