title: Result set with triple union
body: <p>lets say I have a simplified <code>table</code> with 2 Columns:</p>
  
  <pre><code>+------+------+
  | Col1 | Col2 |
  +------+------+
  | a    | b    |
  | a    | c    |
  | b    | d    |
  +------+------+
  </code></pre>
  
  <p>Now, I want a result set where I can wirte my header in line 1 and all distinct values of <code>Col1</code>and <code>Col2</code> in the follwing lines like this:</p>
  
  <pre><code>Header
  a
  b
  c
  d
  </code></pre>
  
  <p>while it works fine with only one Column:</p>
  
  <pre><code>select 'Header'
  union
  select Col1
  from table
  </code></pre>
  
  <p>suprisingly it does not work with more than one union:</p>
  
  <pre><code>select 'Header'
  union
  select Col1
  union
  select Col2
  from table
  </code></pre>
  
  <p>fails. Who knows why and can tell me how to achieve what I want?</p>
  
  <p>Thanks very much!</p>