title: Pick row that has multiple other rows in adjacent table
body: <p>I have a table called <code>conversations</code>. Beside that, I have a table called <code>conversation_participants</code>.</p>
  <p>When users create a conversation, they pick which other users they want to make it with. I do, tough, want to check before creating a new conversation, that it doesn't already exists with that batch of participants - if so I just want to reuse the existing one.</p>
  <p>So my question is. How do I find a row in <code>conversation</code> that has the exact relations in <code>conversation_participants</code>.</p>
  <p>My table looks like this (simplified).</p>
  <pre><code>   | conversations       |
     | ------------------- |
     | id      | int (pk)  |
     | created | timestamp |
     | ------------------- |
  </code></pre>
  <pre><code>   | conversation_participants                |
     | ---------------------------------------- |
     | id           | int (pk)                  |
     | created      | timestamp                 |
     | conversation | int (fk to conversations) |
     | user         | int (fk to users)         |
     | read         | timestamp                 |
     | ---------------------------------------- |
  </code></pre>
  <p>Now the question is. How do I find the row in <code>conversation</code> that has the exact set of users in <code>conversation_participants</code>? It must be the exact set – not just a subset.</p>