Difference database sort null in timestamp column in difference way:

MySQL: NULL values are presented first, or last if you specify DESC to sort in descending order. Exception: In MySQL 4.0.2 through 4.0.10, NULL values sort first regardless of sort order. - http://dev.mysql.com/doc/mysql/en/problems-with-null.html

SQL server 2000, Sybase 12.5 and hsql 1.8.0: NULL values are presented first, or last if you specify DESC to sort in descending order.

Oracle 8: NULL values are presented last, or first if you specify DESC to sort in descending order.

A more detail default sorting behaviour:
{null, -1, +1} // ASC sort for informix, microsoft, mysql and sybase
{-1, +1, null} // ASC sort for IBM, Ingres, InterBase, Oracle and PostgreSQL
{+1, -1, null} // ASC sort for informix, InterBase, PostgreSQL, microsoft, mysql and sybase 
{null, +1, -1} // ASC sort for IBM, Ingres and Oracle

A discussion about using NULL or not - http://discuss.joelonsoftware.com/default.asp?design.4.403282 http://discuss.joelonsoftware.com/default.asp?design.4.494249

One solution - http://web.onetel.com/~hughdarwen/TheThirdManifesto/Missing-info-without-nulls.pdf

Null, mean not application or no value? - http://www.livejournal.com/users/djberg96/47074.html

Nothing neither equals, nor unequals, to NULL - http://fishbowl.pastiche.org/2009/01/30/why_null_is_special/