借书、还书的触发器怎么写？

create table BOOKINFO
(
   BOOKNO               varchar(20)                    not null,
   BOO_BOOKTYPENO       varchar(10)                    not null,
   BOOKNAME             varchar(50)                    not null,
   BOOKAUTHOR           varchar(20)                    not null,
   BOOKINDATE           date                           not null,
   BOOKISBN             varchar(15)                    not null,
   BOOKPRICE            numeric(50),
   constraint PK_BOOKINFO primary key (BOOKNO)
);

create table BOOKTYPE
(
   BOOKTYPENO           varchar(10)                    not null,
   BOOKTYPENAME         varchar(50)                    not null,
   constraint PK_BOOKTYPE primary key (BOOKTYPENO)
);

create table BORROW_RETURNINFO
(
   REA_READNO           varchar(15)                    not null,
   BOO_BOOKNO           varchar(20)                    not null,
   BORROWDATE           date,
   RETURNDATE           date,
   constraint PK_BORROW_RETURNINFO primary key clustered (REA_READNO, BOO_BOOKNO)
);

create table BUYBOOK
(
   PUB_PUBLISHNO        varchar(10)                    not null,
   BOO_BOOKNO           varchar(20)                    not null,
   QUANTITY             numeric(50),
   constraint PK_BUYBOOK primary key clustered (PUB_PUBLISHNO, BOO_BOOKNO)
);

create table PUBLISHINFO
(
   PUBLISHNO            varchar(10)                    not null,
   PUBLISHNAME          varchar(50)                    not null,
   PUBLISHADDRE         varchar(50),
   PUBLISHZIPCODE       varchar(7),
   PUBLISHPHONE         varchar(11),
   constraint PK_PUBLISHINFO primary key (PUBLISHNO)
);

create table READINFO
(
   READNO               varchar(15)                    not null,
   REA_READTYPENO       varchar(10)                    not null,
   READNAME             varchar(30)                    not null,
   READSEX              varchar(4),
   READDATE             date,
   READPHONE            varchar(11),
   READCONDITION        varchar(4),
   constraint PK_READINFO primary key (READNO)
);

create table READTYPE
(
   READTYPENO           varchar(10)                    not null,
   READTYPENAME         varchar(50)                    not null,
   BOOKNUMBER           numeric,
   BOOKDAY              numeric,
   constraint PK_READTYPE primary key (READTYPENO)
);