title: How to add a value to existing value using SQL query
body: <p>I have a table named <code>products</code>.</p>
  <pre><code>Products
      Quantity
      5
  </code></pre>
  <p>I need to update <code>Quantity</code> by any value; for example, adding 3 to the current value, giving output like below:</p>
  <pre><code>Quantity
  8
  </code></pre>
  <p>How can I write an SQL query to accomplish this?</p>