Drupal add distinct filtering for a single field of a Views database query.

gistfile1.php

<?php
/**
 * Implements hook_query_alter().
 * 
 * This hook will make it so your Drupal views are filtered by unique on a single field value.
 * 
 * Make sure to tag your views query first by going to 'Query Settings: settings' configuration
 * in Views UI and, under 'Query Tags', adding 'MY_AWESOME_TAG' or whatever you're using.'
 */
function hook_query_alter(QueryAlterableInterface $query) {
  if ($query->hasTag('MY_AWESOME_TAG')) {
    $query->addJoin('INNER', 'field_data_my_unique_field', 'un', 'node.nid = un.entity_id');

    $query->groupBy('un.field_my_unique_field_value');
  }
}