title: Combine strings to select column
body: <p>I have a table like this</p>
  
  <pre>
  books
  |---------------------|------------------|------------------|------------------|
  |         book        |       source     |    store_price   |  bookstore_price |
  |---------------------|------------------|------------------|------------------|
  |        Batman       |       store      |       10.5       |        9.5       |
  |---------------------|------------------|------------------|------------------|
  |       Superman      |     bookstore    |       null       |         9        |
  |---------------------|------------------|------------------|------------------|
  </pre>
  
  <p>I need to select books and sum prices and my query so far looks like this</p>
  
  <pre><code>SELECT
      book,
      SUM(
          CASE
              WHEN source = 'store' then store_price
              WHEN source = 'bookstore' then bookstore_price
          END
      ) AS price
  FROM books
  GROUP BY 1
  ;
  </code></pre>
  
  <p>Is it possible to somehow optimize SUM select for ex. <code>select source||'_price' from books;</code>?</p>
  
  <p>Thank you in advance!</p>