title: How to understand `u=r÷s`, the division operator, in relational algebra?
body: <p>let be a database having the following relational-schemes: <code>R(A,B,D)</code> and <code>S(A,B)</code> with the attributes of same name in the same domain and with the instances <code>r</code> and <code>s</code> respectively.</p>
  <p>An instance of <code>r</code>
  <a href="https://i.stack.imgur.com/lEjwN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lEjwN.png" alt="an instance of r" /></a></p>
  <p>An instance of <code>s</code></p>
  <p><a href="https://i.stack.imgur.com/L2em7.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/L2em7.jpg" alt="an instance of s" /></a></p>
  <p>What is the scheme and what are the tuples of <code>u=r÷s</code>? How to define them in English with <code>r</code> and <code>s</code>?</p>
  <h1>My attempt</h1>
  <p>I know that</p>
  <p><code>u=r÷s</code>=<a href="https://i.stack.imgur.com/cmmPX.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cmmPX.jpg" alt="u=r÷s" /></a></p>
  <p>Which leads me to think that it would only be an array of one column A, but I'm not sure enough to know what will be ther result within the array.</p>
  <p><strong>Can you help me understand u=r÷s?</strong></p>