mysql触发器

DELIMITER //

CREATE TRIGGER borrow_insert_2
AFTER INSERT ON borrow
FOR EACH ROW
BEGIN
DECLARE num INT;
SET num=SELECT num FROM book WHERE bookId =NEW.bookId;
IF num=0 THEN
ROLLBACK ;
ELSE
UPDATE book SET num=num-1 WHERE bookId=NEW.bookId ;
END IF ;
END //

DELIMITER ;

为什么一直报错
1 queries executed, 0 success, 1 errors, 0 warnings

查询：set num=SELECT num FROM book WHERE bookId =NEW.bookId; IF num=0 THEN ROLLBACK ; ELSE UPDATE book SET num=num-1 WHERE bookId=NEW....

错误代码： 1064
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT num FROM book WHERE bookId =NEW.bookId;
IF num=0 THEN
ROLLBACK ;
EL' at line 1

执行耗时 : 0 sec
传送时间 : 0 sec
总耗时 : 0 sec

回答

问题补充：

用的sqlyog社区版