title: select distinct pair values mysql
body: <p>i have the following table and I'm hoping to extract all the unique pairs for ex: </p>
  
  <pre><code>+------+------+
  | a    | b    |
  +------+------+
  |    1 |    2 |
  |    1 |    3 |
  |    1 |    6 |
  |    1 |    9 |
  |    2 |    1 |
  |    2 |    3 |
  |    3 |    1 |
  |    3 |    2 |
  |    4 |    1 |
  |    6 |    5 |
  +------+------+
  </code></pre>
  
  <p>the out put should be</p>
  
  <pre><code>+------+------+
  | a    | b    |
  +------+------+
  |    1 |    2 |
  |    1 |    3 |
  |    1 |    6 |
  |    1 |    9 |
  |    2 |    3 |
  |    6 |    5 |
  +------+------+
  </code></pre>