Mysql 仅当学生的总学分小于30分时，才创建一个触发器，在studies（rollno，code）表中输入值

我得到一个错误：#1064-您的SQL语法有错误；请查看与MySQL服务器版本对应的手册，以获取第5行“”附近要使用的正确语法

    CREATE TRIGGER db BEFORE INSERT ON studies
    FOR EACH ROW
    BEGIN
    IF((select CAST(SUM(credits) as UNSIGNED) from ((SELECT credits from subject WHERE subject.code = NEW.code) union all (SELECT credits from subject,(SELECT code from studies where studies.rollno = NEW.rollno) as t1 WHERE subject.code = t1.code)) as t2) > 30) THEN
    set NEW.rollno = NULL;
    ENDIF;
    END

---

试试这个-set NEW.rollno=NULL，试试这个-set NEW.rollno=NULL，