SQL> create table tb1 (id varchar2(5) primary key);

Table created


SQL> create table tb2 (id varchar2(4));

Table created

注意tb2的id字段宽度比tb1的要少1。

建立触发器：
create or replace trigger "TR_TB1_INSERT"
after insert on tb1 for EACH ROW

begin
insert into tb2 values (:new.id);
end;
/

tb1插入4个字符，不报错，触发器正常运行：

SQL> insert into tb1 values ('ABCD');

1 row inserted


SQL> commit;

Commit complete


SQL> select * from tb2;

ID
----
ABCD

tb1插入5个字符, 报错，触发器未能正常运行：

SQL> insert into tb1 values ('EFGHI');
insert into tb1 values ('EFGHI')

ORA-12899: value too large for column "XXX"."TB2"."ID" (actual: 5, maximum: 4)
ORA-06512: at "XXX.TR_TB1_INSERT", line 2
ORA-04088: error during execution of trigger 'XXX.TR_TB1_INSERT'

SQL> select * from tb1;

ID
-----
ABCD

SQL> select * from tb2;

ID
----
ABCD

改写触发器，加入异常处理：
create or replace trigger "TR_TB1_INSERT"
after insert on tb1 for EACH ROW

begin
insert into tb2 values (:new.id);
exception
when others then
null;
end;

tb1插入5个字符，不再报错，但触发器未能实现其功能，即tb2未插入相应记录：
SQL> insert into tb1 values ('JKLMN');

1 row inserted


SQL> commit;

Commit complete


SQL> select * from tb2;

ID
----
ABCD

SQL> select * from tb1;

ID
-----
ABCD
JKLMN

触发器用before，然后调用带自治事务的存储过程