title: What does the multiply operator do relational algebra?
body: <p>I'm new to relational algebra. I found the <code>*</code> operator in the following expression</p>
  
  <p><img src="https://i.stack.imgur.com/cof8S.png" alt="enter image description here"></p>
  
  <p>What's the different this and one using join </p>
  
  <p><img src="https://i.stack.imgur.com/SGeLk.png" alt="enter image description here"></p>