public function GroupByClause()
{
$this->match(Lexer::T_GROUP);
$this->match(Lexer::T_BY);
$groupByItems = array($this->GroupByItem());
while ($this->_lexer->isNextToken(Lexer::T_COMMA)) {
$this->match(Lexer::T_COMMA);
$groupByItems[] = $this->GroupByItem();
}
return new AST\GroupByClause($groupByItems);
}