在Oracle的触发器语法中，新插入的记录由:new而不是new引用(请注意冒号).另外，SET是更新语句的一部分，而不是设置字段值的方法-这些是通过简单的赋值完成的，但是请注意，这些是通过:=而不是=完成的.
因此，您的触发器应显示为:

In Oracle's trigger syntax the newly inserted record is referred to by :new, not new (notice the colon). Additionally, SET is a part of an update statement, not a way to set field values - those are done by simple assignments, but note that these are done with := rather than =.
So, your trigger should read:

CREATE OR REPLACE TRIGGER NumberOfBooks
    BEFORE INSERT
    ON book
    FOR EACH ROW
BEGIN
    IF :new.nobook < 10
    THEN
        :new.nobook := 10;
    END IF;
END;