2、根据教材案例学生-课程数据库，为选课表(SC)创建一个INSERT和UPDATE触发器。当试图插入和修改SC表中的记录时，检查Sno（学号）是否在学生表(student)中存在，Cno是否在课程表(Course)中存在。

好的，针对您的问题，我可以为您提供如下的触发器代码：

插入触发器：

CREATE TRIGGER insert_sc_trigger
BEFORE INSERT ON SC
FOR EACH ROW
BEGIN
    DECLARE sno_exist INT;
    DECLARE cno_exist INT;
    SELECT COUNT(*) INTO sno_exist FROM student WHERE Sno = NEW.Sno;
    SELECT COUNT(*) INTO cno_exist FROM Course WHERE Cno = NEW.Cno;
    IF sno_exist = 0 THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Sno does not exist in the student table';
    END IF;
    IF cno_exist = 0 THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cno does not exist in the Course table';
    END IF;
END;

更新触发器：

CREATE TRIGGER update_sc_trigger
BEFORE UPDATE ON SC
FOR EACH ROW
BEGIN
    DECLARE sno_exist INT;
    DECLARE cno_exist INT;
    SELECT COUNT(*) INTO sno_exist FROM student WHERE Sno = NEW.Sno;
    SELECT COUNT(*) INTO cno_exist FROM Course WHERE Cno = NEW.Cno;
    IF sno_exist = 0 THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Sno does not exist in the student table';
    END IF;
    IF cno_exist = 0 THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cno does not exist in the Course table';
    END IF;
END;

以上代码会在插入和更新SC表的记录时触发，首先会检查Sno和Cno是否在student和Course表中存在，如果不存在，则会抛出错误信息。这样可以保证SC表中的学号和课程号都是有效的，且与student和Course表中的记录相匹配。