Have you ever run across the suggestion that you should always use NOT NULL when creating a database table? What is NULL, anyway? Why would you want to use it? What are the advantages to using NULL? How does NULL figure in to LEFT OUTER joins?
Rudy answers these questions with plenty of examples in this article about NULL. The advantage to using NULL is described in the context of designing database tables where a column's value can be unknown or not appropriate. NULL is also discussed in the use of aggregate functions. Finally, an illustration of NULL in LEFT OUTER joins is provided.
Should you use NULL? This article answers the question positively.