title: sql add user input to existing value
body: <p>I'm using a psql database and the rest is Ruby using Sinatra.
  I've got my controllers, my models, my views, etc.</p>
  
  <p>I'm doing a little inventory app and currently I can only update the database with the new quantity in stock via a html form in one of my views.</p>
  
  <pre><code>UPDATE products SET (name, quantity) = ('#{@name}', #{@quantity}) WHERE id = #{@id};
  </code></pre>
  
  <p>However, I'd like to be able to put in the amount that was ordered/delivered into a form instead of the new quantity on stock.</p>
  
  <p>I know I can do</p>
  
  <pre><code>UPDATE product SET quantity = quantity + 200 WHERE...and so on.
  </code></pre>
  
  <p>but that doesn't take the 200 from the form.</p>
  
  <p>This is the first time I'm asking a question here, so if there is anything missing please let me know and I'll provide you with more info.</p>