title: Why does Salesforce consider this simple parent-to-child relationship SOQL query not valid?
body: <p>A CaseMilestone record can have one Case record and one Case record can have many CaseMilestone records.</p>
  
  <p>So this works:</p>
  
  <pre><code>SELECT Id, (SELECT Id FROM CaseMilestones) FROM Case
  </code></pre>
  
  <p>Similarly, a CaseMilestone record can have one MilestoneType record and one MilestoneType record can have many CaseMilestone records, but the following doesn't work:</p>
  
  <pre><code>SELECT Id, (SELECT Id FROM CaseMilestones) FROM MilestoneType
  </code></pre>
  
  <p>How come? If it is because not all CaseMilestones are required to have an associated MilestoneType, which query will get me what I'm looking for?</p>