title: How to get first date in series of versions
body: <p>I have a table "t1"</p>
  
  <pre><code>| Version | Date    | successor |
  ---------------------------------
  | a       | 'day 1' | c         |
  | b       | 'day 2' | e         |
  | c       | 'day 3' |           |
  | d       | 'day 4' |           |
  | e       | 'day 5' | f         |
  | f       | 'day 6' |           |
  </code></pre>
  
  <p>and I want to query every version with the earliest date in the series of versions (a series consists of a version and its successor - multiple chains possible). In this case it should output.</p>
  
  <pre><code>| Version | Date    | 
  ---------------------
  | a       | 'day 1' |
  | b       | 'day 2' |
  | c       | 'day 1' |
  | d       | 'day 4' |
  | e       | 'day 2' |
  | f       | 'day 2' |
  </code></pre>
  
  <p>I already tried it with a left join query of the same table, but in my case it outputted two dates in separate columns, but not one in one column (the earliest).</p>
  
  <p>How does that work?</p>
  
  <p>(Good solutions based on better table-design are also appreciated)</p>