Oracle 11g Release 2

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create table tab ( id int primary key );
insert into tab select level from dual connect by level <= 10;
10 rows affected

declare
  nmr tab.id%type;
  ext pls_integer := 0;
begin
  select max(id) into nmr from tab;
  begin
     select 1 into ext from user_sequences s where regexp_like(sequence_name,'myseq','i');
   exception when no_data_found then ext := 0;
  end;
  if ext = 0 then
     execute immediate 'create sequence myseq';
  end if;
     execute immediate 'alter sequence myseq increment by '||nmr;
     execute immediate 'select myseq.nextval from dual' into nmr;
     execute immediate 'alter sequence myseq increment by 1';
end;
/
1 rows affected

select myseq.nextval from dual


NEXTVAL


11


select myseq.nextval from dual


NEXTVAL


12