Before trigger processing multiple rows and just the current one

[daljitsb]

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE 11.2.0.1.0 Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

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What am I doing wrong. Why it is inserting 3 records when it should only 1)? Please advice. Thanks
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I have a before trigger that is processing (inserting in this case) multiple previous rows along with the current one. To debug, I made it very basic one but it is still doing the same thing.

Z_table.message_id is the primary key.


Please see the trigger code below:
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create table Z_table_copy as select * from z_table where 1=2;

truncate table Z_table_copy ;


create or replace trigger tr_new
before insert or update or delete
on z_table
REFERENCING OLD AS OLD NEW AS NEW
FOR EACH ROW
when ( NEW.group_code = 'abc')
declare

v_group_id integer;

begin

Insert into Z_table_copy (message_id, change_id, group_code, comments)
Values(:NEW.message_id, :NEW.change_id, :NEW.group_code, :NEW.comments);


end;


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insert into Z_table Values ('aa', 1, 'abc', 'mycomment')

Select count(*) from Z_table_copy ;

count(*)
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3