这将使您更近一些：

CREATE TABLE tblEnrolled (
    id INT(11) AUTO_INCREMENT PRIMARY KEY,studNum VARCHAR(9),subjCode VARCHAR(20)
);

DELIMITER $$

CREATE PROCEDURE grade(IN _studNum VARCHAR(9),IN _subjCode VARCHAR(20))
    RETURNS VARCHAR (50)
    DETERMINISTIC
BEGIN
    IF (_studNum IS NULL)
    THEN -- a. If the student number does not exist,do not insert,SELECT "Student Number not supplied.";
    ELSE IF (_subjCode IS NULL)
    THEN  --  b. If the subject code does not exist,response: 
        SELECT "Subject Code does not exist.";
    ELSE IF ( EXISTS( SELECT 1 FROM tbl WHERE ... ) )
    THEN -- c. If the student number is already graded in the given subject code with passing grade (including INC,4.00),response: 
        SELECT "Student already passed the given subject.";
    ELSE BEGIN
        SET @sql = "SELECT grade INTO @grade FROM tblGrade WHERE student_num = ?";
        PREPARE _SQL FROM @sql;
        EXECUTE _SQL,_student_num;
        DEALLOCATE PREPARE _SQL;
        IF @grade IS NULL OR @grade < 1.0
        THEN  -- d. If the student number and subject code do not exist in the tblGrade table or it is in the tblGrade table but with failing grade,insert the record,BEGIN
                INSERT INTO ...;
                SELECT "New record added.";
            END;
        END IF;
    END;
END $$

DELIMITER ;