A good _QueryTable class?

table.php

<?php

    class _QueryTable extends _QueryTableExpression {
        private $identifier;

        function __construct($identifier, $alias = null) {
            $this->identifier = _QueryIdentifier::create($identifier, $alias);
        }

        public static function create($identifier, $alias = null) {
            // this is probably a self-join that's aliased.
            if ($alias) {
                return new _QueryTable($identifier, $alias);
            }

            // from here on out, don't bother checking for aliases.

            // any table expression is a stand-in for a table
            if (is_a($identifier, '_QueryTableExpression')) {
                return $identifier;
            }

            // pass off handling to the constructor
            else {
                return new _QueryTable($identifier);
            }
        }

        public function sql_string() {
            return $this->do_alias($this->identifier->sql_string());
        }
    }

?>