title: Why is a primary-foreign key relation required when we can join without it?
body: <p>If we can get data from two tables without having primary and foreign key relation, then why we need this rule? Can you please explain me clearly, with suitable example?
  It's a test database, don't mind the bad structure. </p>
  
  <p>Tables' structure:</p>
  
  <p>**</p>
  
  <pre><code>table - 'test1'
  columns - id,lname,fname,dob
  no primary and foreign key and also not unique(without any constraints)
  </code></pre>
  
  <p>**</p>
  
  <pre><code>**table - 'test2'
  columns- id,native_city
  again, no relations and no constraints** 
  </code></pre>
  
  <p>I can still join these tables with same columns 'id',
  so if there's no primary-foreign key, then what is the use of that?</p>