title: Select distinct from multiple columns and its matching values sum
body: <p>I have this table named <code>Orders</code>. 
  Each row of the table represents an order made by a customer.</p>
  
  <p><strong>prod</strong> means <strong>product</strong></p>
  
  <pre><code>+-----------------------------------------------------------------------------------+
  | prod_1      | prod_1_qty  | prod_2      |prod_2_qty   | prod_3      | prod_3_qty  |
  |-------------------------------------------------------|---------------------------|
  | chair       | 3           | board       |9            | bed         |4            |
  | board       | 8           | door        |2            | desk        |2            |
  | chair       | 2           | window      |1            | door        |6            |
  | desk        | 4           | chair       |3            | sofa        |1            |
  </code></pre>
  
  <p>I would like to write a query that returns the quantity of each <code>product</code> ordered like this:</p>
  
  <pre><code>+---------------------------+
  | product     | product_qty |
  |---------------------------|
  | chair       | 8           |
  | board       | 17          |
  | door        | 8           |
  | window      | 1           |
  | sofa        | 1           |
  | bed         | 4           |
  | desk        | 6           |
  </code></pre>
  
  <p>Is there any way to achieve this using <code>T-SQL</code>, and if so, what is the query one would use to do this?</p>