mysql存储过程传参应注意的问题

DROP PROCEDURE IF EXISTS avgBookstar;
DELIMITER $$
CREATE PROCEDURE avgBookstar(IN selectBookID CHAR(50),OUT selectBookStar INT)
BEGIN
SELECT AVG(star) FROM t_comment,t_orderitem,t_book
WHERE t_orderitem.book_ID = t_book.bookID
AND t_orderitem.orderItemID = t_comment.orderItem_ID
AND t_book.bookID = selectBookID
INTO selectBookStar;
SELECT selectBookStar;
END $$
DELIMITER ;

CALL avgBookstar('111111',@star);
SELECT @star;