Instead-of触发器

create trigger CheckId
insetead of update     --替换原来的update
as
if not exists(select * from student where id=:new.id)
    begin
        rollback transaction
        DBMS_OUTPUT.PUT_LINE(‘要处理记录的学生ID不存在‘);
    end;
else
    begin
    update student
    set first_name=:new.first_name;
    where id= :new.id;
    end;
end CheckId;
/

--触发 trigger CheckId
update student
set first_name=‘David‘
where id=‘12721205‘     --其实没有实现这个update，而是实现了CheckId的主体

create or replace view books_authors as
    select b.isbn,b.title,a.first_name,a.last_name
    from books b
    join authors a
    on b.authors_id1=a.id
    or b.authors_id2=a.id
    or b.authors_id3=a.id;

insert into books_authors(isbn,title,first_name,last_name)
vaules(‘72223855‘,‘SQL Cookbook‘,‘Joe‘,‘Blow‘);

create or replace trigger InsertBooksAuthors
    instead of insert on books_authors   --这个insert 替换了前面的insert，它没有直接更新不可更新视图，而是更新生成它的两个底层表
declare
    v_book books%rowtype;
    v_author_id authors.id%type;
begin
    begin
        select id
        into v_author_id
        from authors
        where first_name= :new.first_name
        and last_name= :new.last_name;    --如果插入的记录(first_name,last_name 在books表中存在，就取出这条记录)
    exception
        when no_data_found then
        insert into authors(id，first_name,last_name)
        vaules(author_sequence.nextval,:new.first_name,:new.last_name)
        returning id into v_author_id;   --如果不存在则重新插入到authors表
    end;

    select *
    into v_book
    from books
    where isbn= :new.isbn;

    if v_book.author_id2 is null then
        update books
        set author_id2=v_author_id
        where isbn= :new.isbn;
    else
        update books
        set author_id3=v_author_id
        where isbn= :new.isbn;
    end if;
end InsertBooksAuthors;