$items = array('A', 'B', 'C', 'D');

/* Database contains any letters between A..Z. Could contain none, all, or any combination of letters, no duplication */

// Is it more efficient to:

foreach ($items as $item)
{
	$sql = "SELECT 1
		FROM letters
		WHERE letter = $item";
	$result_set = $dbObj->execute_query($sql);
	if (!$dbObj->get_num_rows($result_set))
	{
		// item is not in the database
		// ...
	}
}

// ... or ...

$sql = "SELECT letter
	FROM letters";
$result_set = $dbObj->execute_query($sql);
$item_array = $dbObj->get_row_array($result_set);

foreach ($items as $item)
{
	if (!in_array($item, $item_array))
	{
		// item is not in database
		// ...
	}
}