title: Understanding of cardinality it ERD models
body: <p>I have huge trouble with understanding the cardinality.</p>
  
  <p>Let's say that there Doctor and he can perform heart surgery on just one patient. Surely there are many doctors and many patients, but should the model be just like this?</p>
  
  <pre><code>PACIENT-|----------|- DOCTOR   meaning 1:1?
  </code></pre>
  
  <p>Does it not need to reflect that there are many doctors and many patients? Or this describes relationship between two actual instances?</p>
  
  <p>Another example:
  Hospital runs an experiment and can have many patients in drug trials. Each trial tests just one drug and no patient can appear in more than one trial.</p>
  
  <p><code>TRIAL |- ------&lt;-PATIENT</code> (MANY TO ONE - more patients in trials are allowed)</p>
  
  <p><code>PATIENT|------|-DRUG</code>   (1:1 if one patient can only take one drug)?</p>
  
  <p>But how to indicate that the same patient cannot appear in any other trial? Or is it implicit by the relations between Trial and Patient where there is 1 at the side of Trial?</p>