title: MySQL, Join two rows from one table and ignore doublons
body: <p>I have a table "user" like this : </p>
  
  <pre><code>| uid | username |
  |  1  | Jack     |
  |  2  | John     |
  |  3  | Robert   |
  </code></pre>
  
  <p>Using my request : </p>
  
  <pre><code>SELECT u1.username, u2.username FROM user u1 JOIN user u2
  </code></pre>
  
  <p>Using this request, I get a table like this : </p>
  
  <pre><code>| uid | username | uid | username |
  |  1  | Jack     |  1  | Jack     |
  |  1  | Jack     |  2  | John     |
  |  1  | Jack     |  3  | Robert   |
  |  2  | John     |  1  | Jack     |
  |  2  | John     |  2  | John     |
  |  2  | John     |  3  | Robert   |
  |  3  | Robert   |  1  | Jack     |
  |  3  | Robert   |  2  | John     |
  |  3  | Robert   |  3  | Robert   |
  </code></pre>
  
  <p>How can I remove one of these duets?</p>
  
  <pre><code>|  1  | Jack     |  2  | John     |
  |  2  | John     |  1  | Jack     |
  </code></pre>