Add a auto increment primary key to existing table in oracle [duplicate]

Say your table is called t1 and your primary-key is called id
First, create the sequence:

create sequence t1_seq start with 1 increment by 1 nomaxvalue; 

Then create a trigger that increments upon insert:

create trigger t1_trigger
before insert on t1
for each row
   begin
     select t1_seq.nextval into :new.id from dual;
   end;