title: SQL Instruction - set sum, product,
body: <p>how to write sql instruction for: </p>
  
  <ol>
  <li>r U s</li>
  <li>r ∩ s</li>
  <li>r - s</li>
  <li>s - r </li>
  </ol>
  
  <p>?
  i can not find enything about how to write this :(</p>