public function generateAlias(Clause $clause)
    {
        if ($clause instanceof FieldName)
        {
            $table = $clause->getTable();
            if (empty($table))
            {
                // When no table is defined, it means only one table is being
                // used in the query. Aliases will not be required
                return "";
            }

            $prefix = $table->getPrefix();
            $alias = $prefix . '_' . $clause->getField();
        }
        elseif ($clause instanceof SQLFunction)
        {
            $func = $clause->getFunction();
            $alias = strtolower($func);
        }
        else
            throw new ImplementationException("No alias generation implemented for: " . get_class($clause));

        $cnt = 1;
        $base_alias = $alias;
        while (isset($this->field_aliases[$alias]))
            $alias = $base_alias . (++$cnt);

        $this->field_aliases[$alias] = true;
        return $alias;
    }