选择触发时机：before||after
选择DML触发：insert、update、delete
触发模式：
for each row–单行触发；
或者执行完触发
--更新表时，输出更新前后salary的值
create or replace trigger update_emp_trigger
after
--before or after
update on employees
for each row
--each or whole
begin
dbms_output.put_line('old salary'||','||:old.salary||','||'new salary'||','||:new.salary);
end;
--删除表数据时，将其备份到bak表
create or replace trigger delete_emp_bak
before
delete on my_emp
for each row
begin
insert into my_emp_bak
values(:old.employee_id,:old.salary);
end;
