title: How to "reverse" a simple 2 column relationship table
body: <p>Good evening,</p>
  
  <p>I'm new to SQL.</p>
  
  <p>I've been trying to "reverse" a  a simple 2 column relationship table.</p>
  
  <p>Here is an example to explain it :</p>
  
  <p>TABLE KnowEachOther</p>
  
  <pre><code>¦     id_human1   ¦     id_human2      ¦
  ¦        1        ¦         10         ¦
  ¦        1        ¦         11         ¦
  ¦        2        ¦         12         ¦
  ¦        2        ¦         13         ¦
  </code></pre>
  
  <p>So here I have a table of people that know each other. Each line means humanX and humanY know each other. Now I want to get a table of people that don't know each other ( assuming this table has all the people ). This would give this :</p>
  
  <p>TABLE DontKnowEachOther</p>
  
  <pre><code>¦     id_human1   ¦     id_human2      ¦
  ¦        1        ¦          2         ¦
  ¦        1        ¦         12         ¦
  ¦        1        ¦         13         ¦
  ¦        2        ¦         10         ¦
  ¦        2        ¦         11         ¦
  ¦        11       ¦         10         ¦
  ¦        12       ¦         10         ¦
  ¦        13       ¦         10         ¦
  ¦        11       ¦         12         ¦
  ¦        11       ¦         13         ¦
  ¦        12       ¦         13         ¦
  </code></pre>
  
  <p>Any hints on how to do that would be appreciated. What would be even more appreciated is how you approach such non-trivial task. Trying to get better here ;).</p>
  
  <p>Many thanks</p>