/**
* Creates an instance of binary expression node, representing the construction: subject IS NULL
*
* @param mixed $subject logical subject
*
* SQL example:
* @code
* // "id" IS NULL
* Expression::isNull("id");
* @endcode
* @return UnaryPostfixExpression
*/
static function isNull($subject)
{
return new UnaryPostfixExpression($subject, UnaryPostfixLogicalOperator::isNull());
}
