title: How to query using ← / back arrow / asignment / naming / renaming / declaration
body: <blockquote>
    <p>Student(id, name, address, city, birthYear)<br>
    Course(code, courseName, lengthWeeks, institution°)<br>
    Institution(instNr, instName, instCity)<br>
    Takes(id°, code°)  </p>
    
    <p>List the ids and names of students who do not take the Databases course.</p>
    
    <p>t1 ← σ courseName=Databases (Course)<br>
    t2 ← t1 ⋈ Takes<br>
    t3 ← t2 ⋈ Student<br>
    t4 ← π id, name (t3)<br>
    t5 ← π id, name (Student)<br>
    R ← t5 \ t4  </p>
  </blockquote>
  
  <p>I don't understand the logic behind t5. How can you pick id's and names that are already removed from the relation? Could you do it another way with the ¬?</p>