title: What is BUCKET_ID in MediaStore?
body: <p>I was browsing through some code that queries the MediaStore.Images content provider and uses a column named BUCKET_ID:</p>
  
  <pre><code>Uri uri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
  String projection[] = {
      "DISTINCT " + MediaStore.Images.Media.BUCKET_DISPLAY_NAME,
      MediaStore.Images.Media.BUCKET_ID,
      MediaStore.Images.Media.DATE_TAKEN,
      MediaStore.Images.Media.DATA
  };
  String BUCKET_GROUP_BY = "1) GROUP BY 1,(2";
  String BUCKET_ORDER_BY = "MAX(datetaken) DESC";
  Cursor imagecursor = getContentResolver().query(uri, projection, BUCKET_GROUP_BY, null, BUCKET_ORDER_BY);
  </code></pre>
  
  <p>In general, what do these columns represent? <strong>Can anyone explain with examples?</strong>(<em>The docs have a one-liner about these columns that explains quite less</em>)</p>