title: Where clause if there are multiple of the same ID
body: <p>I have following table:</p>
  
  <pre><code>ID | source | Name | Age | ... | ...
  1  | SQL    | John | 18  | ... | ...
  2  | SAP    | Mike | 21  | ... | ...
  2  | SQL    | Mike | 20  | ... | ...
  3  | SAP    | Jill | 25  | ... | ...
  </code></pre>
  
  <p>I want to have one record for each ID. The idea behind this is that if the ID comes only once (no matter the Source), that record will be taken. But, If there are 2 records for one ID, the one containing SQL as source will be the used record here.</p>
  
  <p>So, In this case, the result will be:</p>
  
  <pre><code>ID | source | Name | Age | ... | ...
  1  | SQL    | John | 18  | ... | ...
  2  | SQL    | Mike | 20  | ... | ...
  3  | SAP    | Jill | 25  | ... | ...
  </code></pre>
  
  <p>I did this with a partition over (ordered by Source desc), but that wouldn't work well if a third source will be added one day.</p>
  
  <p>Any other options/ideas?</p>