title: DATABASE_URL is not set(CS50)
body: <p>I'm working on cs50 project1 (js &amp; python), I download the project, 
  and found this line in it:</p>
  
  <pre><code>engine = create_engine(os.getenv("DATABASE_URL"))
  </code></pre>
  
  <p>the question is how to set up the DATABASE_URL? </p>