title: Can row_number() ignore null in oracle
body: <p>I have data like this</p>
  
  <pre><code>---------------------------
  |   code    | other column
  ---------------------------
  |   C       |    a
  |   null    |    a
  |   A       |    a
  |   null    |    a
  |   null    |    a
  ----------------------------
  </code></pre>
  
  <p>How can i write query to get row_number without counting null column.</p>
  
  <pre><code>----------------------------------
  | id  |   code    | other column |
  ----------------------------------
  | 1   |   C       |    a
  |     |   null    |    a
  | 2   |   A       |    a
  |     |   null    |    a
  |     |   null    |    a
  ----------------------------------
  </code></pre>