title: SQL count occurrences of entries
body: <p>I'm having some problems in counting some entries in my database.</p>
  
  <p>I have following query:</p>
  
  <pre><code>SELECT ref_training_date, training_date FROM fynslund.users_training
  INNER JOIN training
  ON training.id = users_training.ref_training_date
  WHERE attendance = 1
  </code></pre>
  
  <p>Which gives me following results: </p>
  
  <pre>
  ┌──────┬───────────────┐
  │ ID   │      DATE     │
  ├──────┼───────────────┤
  │ '55' │  '2018-01-09' │
  │ '55' │  '2018-01-09' │
  │ '54' │  '2018-02-03' │
  │ '54' │  '2018-02-03' │
  │ '54' │  '2018-02-03' │
  │ '54' │  '2018-02-03' │
  └──────┴───────────────┘
  </pre>
  
  <p>How do I count how many times the date with ID <code>'55'</code> appears?</p>