<?php
$queries = array(
  'SELECT node.nid FROM node' => 'SELECT DISTINCT(node.nid) FROM node',
  'SELECT nid FROM node' => 'SELECT DISTINCT(node.nid) FROM node',
  'seLecT nId frOm NoDe' => 'SELECT DISTINCT(node.nid) FROM NoDe',
  'SELECT DISTINCT(yo), nid FROM node' => 'SELECT DISTINCT(yo), DISTINCT(node.nid) FROM node',
  'SELECT nid, DISTINCT(nid) FROM node' => 'SELECT DISTINCT(node.nid), DISTINCT(nid) FROM node',
  'SELECT DISTINCT(nid) FROM node' => 'SELECT DISTINCT(nid) FROM node',
  'SELECT DISTINCT(node.nid) AS nid FROM node' => 'SELECT DISTINCT(node.nid) AS nid FROM node',
  'SELECT something AS nid FROM node' => 'SELECT something AS nid FROM node',
  'SELECT yo_node.nid FROM node' => 'SELECT yo_node.nid FROM node',
  'SELECT something FROM other WHERE j IN (SELECT node.nid FROM node)' => 'SELECT something FROM other WHERE j IN (SELECT node.nid FROM node)',
  'SELECT nid FROM other WHERE j IN (SELECT node.nid FROM node)' => 'SELECT DISTINCT(node.nid) FROM other WHERE j IN (SELECT node.nid FROM node)',
  'SELECT node.nid_ FROM node' => 'SELECT node.nid_ FROM node',
  'SELECT a, nid, b FROM node' => 'SELECT a, DISTINCT(node.nid), b FROM node'
);
  
$count = 0;
foreach ($queries as $o => $f) {
  $r = db_distinct_field('node', 'nid', $o);
  if ($f != $r) {
    echo "Failed for '$o' : got '$r' but expected '$f'";
    $count++;
  }
}

if ($count == 0) {
  echo "All good\n";
}
?>