row level triggers : 是每个row改变时，触发trigger。比如，２个row update，触发２次。

table level triggers: 是table改变时，触发trigger。无论几个row改变都没影响, 比如，1个row update触发1次 ,２个row update，也触发1次。

顺便转个教程

Before / for each row trigger

A before trigger is called before because it fires before the new values (:new.field_name) are stored in the table. That means that the new value can be changed in the trigger.

create table t_update_before_each_row (
txt varchar2(10)
);
create table log (
txt varchar2(20)
); create trigger update_before_each_row before update on t_update_before_each_row for each row begin :new.txt := upper(:new.txt);
insert into log values ('old: ' || :old.txt);
insert into log values ('new: ' || :new.txt); end update_before_each_row;
/

insert into t_update_before_each_row values('one');
insert into t_update_before_each_row values('two');
insert into t_update_before_each_row values('three');
insert into t_update_before_each_row values('four');

Updating (that is: concatenating the value with itself) the rows containing two and three:

update t_update_before_each_row set txt = txt || txt
where substr(txt,1,1) = 't';

select * from t_update_before_each_row;

As can be seen by the output of the select statement, the trigger changed the values of the new values; they're in uppercase now:

one
TWOTWO
THREETHREE
four

The log displays the old and new values:

select * from log;

old: two
new: TWOTWO
old: three
new: THREETHREE

Cleaning up:

drop table t_update_before_each_row;
drop table log;

After / for each row trigger

In contrast to a before trigger, an after trigger does not allow to change :new.field_name because the value is, when the trigger fires, already written to the table.

If one tries to assign a value to :new.field_name, Oracle throws an ORA-04084: cannot change NEW values for this trigger type.

create table t_update_after_each_row (
txt varchar2(10)
);
create table log (
txt varchar2(20)
);
create trigger update_after_each_row after update on t_update_after_each_row for each row begin
-- :new.txt := upper(:old.txt); -- ORA-04084: cannot change NEW values for this trigger type
insert into log values ('old: ' || :old.txt);
insert into log values ('new: ' || :new.txt);
end update_after_each_row;
/

insert into t_update_after_each_row values('one');
insert into t_update_after_each_row values('two');
insert into t_update_after_each_row values('three');
insert into t_update_after_each_row values('four');

update t_update_after_each_row set txt = txt || txt
where substr(txt,1,1) = 't';

select * from t_update_after_each_row;

one
twotwo
threethree
four

select * from log;

As the log table shows, it is possible to use :new and :old although it's not possible to assign something to :new.

old: two
new: twotwo
old: three
new: threethree

Cleaning up:

drop table t_update_after_each_row;
drop table log;

Table level trigger

A table level trigger is a trigger that doesn't fire for each row to be changed. Accordingly, it lacks the for each row. Consequently, both, the :new and:old are not permitted in the trigger's PL/SQL block, otherwise, an ORA-04082: NEW or OLD references not allowed in table level triggers is thrown.

create table t_update_before (
txt varchar2(10)
);
create table log (
txt varchar2(20)
);
create trigger update_before
before update on t_update_before
begin
-- :new.txt := upper(:old.txt); -- ORA-04082 insert into log values ('update trigger');
end update_before;
/

insert into t_update_before values('one');
insert into t_update_before values('two');
insert into t_update_before values('three');
insert into t_update_before values('four');

update t_update_before set txt = txt || txt
where substr(txt,1,1) = 't';

select * from t_update_before;

one
twotwo
threethree
four

Although two rows were updated, only one record is found in the log table:

select * from log;

update trigger

An update statement that doesn't update any row:

update t_update_before set txt = txt || txt
where txt = 'no update';

Still, the trigger fires...

select * from log;

... which results in another row found in the log table:

update trigger
update trigger

Cleaning up:

drop table t_update_before;
drop table log;

Order of execution

Oracle allows to create multiple triggers on the same table. The order of the execution of these triggers is undeterministic (or random, if you want this word) except that all before triggers fire before the after triggers.

ITPub博客

each row and table level triggers

Before / for each row trigger

After / for each row trigger

Table level trigger

Order of execution

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