protected function visitUnaryOperation(UnaryOperationExpression $expression)
{
$operator = $expression->getOperator();
if (!isset($this->unaryOperators[$operator])) {
throw new PinqDemoSqlException("Unary operator not supported: {$operator}");
}
$this->sql .= '(';
$this->sql .= $this->unaryOperators[$operator];
$this->walk($expression->getOperand());
$this->sql .= ')';
}