sql - How to create a trigger which, update a row in the same table after insert? -

for example, have table t_1 , insert id value, trigger should after insert, update value id_2 , set id_2 id:

create table t_1( id number(10), id_1 number(10) );

i create trigger:

create or replace trigger id_to_id_2 after insert on t_1 each row begin update t_1 set id_2=:new.id id = new.id; end;  /

but when try insert, error:

    db constraint error: ora-04091: table t_1is mutating, trigger/function may  not see it\nora-06512: @

i don't understand error, explain me doing wrong?

you need before insert trigger this:

create or replace trigger id_to_id_2 before insert on t_1 each row begin   :new.id_2 := :new.id; end;

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