title: Calculate age based on multiple fields
body: <p>I have a table with a date of birth column, a date of death column and the date the dataset was extracted.</p>
  <p>I want to calculate each person's age as either the date of their death or their age at the date of extract - not by the current date - with a new 'Age' column. I can't quite work it out at the moment.</p>
  <p>Example data:</p>
  <p><strong>Current table</strong></p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th style="text-align: left;">person_id</th>
  <th style="text-align: center;">birth_datetimetime</th>
  <th style="text-align: right;">death_datetime</th>
  <th style="text-align: right;">extract_date</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td style="text-align: left;">1234</td>
  <td style="text-align: center;">1980-04-01T00:00:00</td>
  <td style="text-align: right;">null</td>
  <td style="text-align: right;">2021-12-31</td>
  </tr>
  <tr>
  <td style="text-align: left;">8765</td>
  <td style="text-align: center;">1925-05-04T00:00:00</td>
  <td style="text-align: right;">2018-05-T00:00:00</td>
  <td style="text-align: right;">2021-12-31</td>
  </tr>
  <tr>
  <td style="text-align: left;">9102</td>
  <td style="text-align: center;">1974-05-17T00:00:00</td>
  <td style="text-align: right;">2021-01-31T00:00:00</td>
  <td style="text-align: right;">2021-12-31</td>
  </tr>
  <tr>
  <td style="text-align: left;">5678</td>
  <td style="text-align: center;">2019-09-01T00:00:00</td>
  <td style="text-align: right;">null</td>
  <td style="text-align: right;">2021-12-31</td>
  </tr>
  <tr>
  <td style="text-align: left;">3456</td>
  <td style="text-align: center;">1947-04-01T00:00:00</td>
  <td style="text-align: right;">2016-06-14T00:00:00</td>
  <td style="text-align: right;">2021-12-31</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p><strong>Desired output</strong></p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th style="text-align: left;">person_id</th>
  <th style="text-align: center;">birth_datetimetime</th>
  <th style="text-align: right;">death_datetime</th>
  <th style="text-align: right;">extract_date</th>
  <th style="text-align: right;">Age</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td style="text-align: left;">1234</td>
  <td style="text-align: center;">1980-04-01T00:00:00</td>
  <td style="text-align: right;">null</td>
  <td style="text-align: right;">2021-12-31</td>
  <td style="text-align: right;">41</td>
  </tr>
  <tr>
  <td style="text-align: left;">8765</td>
  <td style="text-align: center;">1925-05-04T00:00:00</td>
  <td style="text-align: right;">2018-05-T00:00:00</td>
  <td style="text-align: right;">2021-12-31</td>
  <td style="text-align: right;">93</td>
  </tr>
  <tr>
  <td style="text-align: left;">9102</td>
  <td style="text-align: center;">1974-05-17T00:00:00</td>
  <td style="text-align: right;">2021-01-31T00:00:00</td>
  <td style="text-align: right;">2021-12-31</td>
  <td style="text-align: right;">47</td>
  </tr>
  <tr>
  <td style="text-align: left;">5678</td>
  <td style="text-align: center;">2019-09-01T00:00:00</td>
  <td style="text-align: right;">null</td>
  <td style="text-align: right;">2021-12-31</td>
  <td style="text-align: right;">2</td>
  </tr>
  <tr>
  <td style="text-align: left;">3456</td>
  <td style="text-align: center;">1947-04-01T00:00:00</td>
  <td style="text-align: right;">2016-06-14T00:00:00</td>
  <td style="text-align: right;">2021-12-31</td>
  <td style="text-align: right;">69</td>
  </tr>
  </tbody>
  </table>
  </div>