$id = 0;
while(true)
{
    $sql = 'select * from table_a  where state=600 '. ($id ? " AND id < $id" : "") .' order by id desc limit 1000';
    $multi_row = $db->query($sql)->get_result();
    if(! $multi_row)
    {
        break;
    }
    foreach($multi_row as  $val)
    {
       $id = $val['id']; // $val->id??? 
    }
}