title: SQL - Select rows with same value in one column and choose the one with specific value in other column
body: <p>I just started learning SQL and ran into a problem that I can't solve on my own.</p>
  <p>I have the following table:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>id</th>
  <th>project</th>
  <th>field</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>10301</td>
  <td>A</td>
  <td>field_10200</td>
  </tr>
  <tr>
  <td>10302</td>
  <td>null</td>
  <td>field_10201</td>
  </tr>
  <tr>
  <td>10303</td>
  <td>A</td>
  <td>field_10202</td>
  </tr>
  <tr>
  <td>10400</td>
  <td>null</td>
  <td>field_10300</td>
  </tr>
  <tr>
  <td>10401</td>
  <td>null</td>
  <td>field_10301</td>
  </tr>
  <tr>
  <td>10500</td>
  <td>null</td>
  <td>field_10400</td>
  </tr>
  <tr>
  <td>10502</td>
  <td>B</td>
  <td>field_10212</td>
  </tr>
  <tr>
  <td>10505</td>
  <td>A</td>
  <td>field_10301</td>
  </tr>
  <tr>
  <td>10506</td>
  <td>B</td>
  <td>field_10301</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Now I want to output only the IDs where the field belongs to project A or null but not both.</p>
  <p>The output should look like this:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>id</th>
  <th>project</th>
  <th>field</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>10301</td>
  <td>A</td>
  <td>field_10200</td>
  </tr>
  <tr>
  <td>10302</td>
  <td>null</td>
  <td>field_10201</td>
  </tr>
  <tr>
  <td>10303</td>
  <td>A</td>
  <td>field_10202</td>
  </tr>
  <tr>
  <td>10400</td>
  <td>null</td>
  <td>field_10300</td>
  </tr>
  <tr>
  <td>10500</td>
  <td>null</td>
  <td>field_10400</td>
  </tr>
  <tr>
  <td>10505</td>
  <td>A</td>
  <td>field_10301</td>
  </tr>
  </tbody>
  </table>
  </div>