create or replace procedure upsert_t23
    ( p_id in t23.id%type
      , p_name in t23.name%type )
is
    cursor c is
        select null
        from t23
        where id = p_id;
    dummy varchar2(1);
begin
    open c;
    fetch c into dummy;
    if c%notfound then
        insert into t23
            values (p_id, p_name);
    else
        update t23
             set name = p_name
             where id = p_id;
    end if;
 end;

SSN1>  exec upsert_t23(100, 'FOX IN SOCKS')

SSN2>  exec upsert_t23(100, 'MR KNOX')

...
lock table t23 in row shared mode nowait;
open c;
...

try {
  // MERGE
}
catch (dup_val_on_index) {
  // UPDATE
}