Recently, this powerful capability has been under a cloud of controversy. Errors and weak support for nulls in ANSI SQL and in commercial RDBMSs have created a general perception that relational nulls are flawed and should be used with extreme care. Two papers in the Issues Section examine this controversy - Should Nulls be considered harmful? and '..., bridges could collapse, spacecraft could fall out of the sky, and so on', Do DBMSs Process Subqueries Correctly?.
Fortunately, the situation is not as bad as it seems. The problem is not with the relational model but with poor implementations. There is an RDBMS that processes nulls fully and correctly - FirstSQL. Unlike virtually all other RDBMSs, FirstSQL has a fundamentally sound implementation of relational nulls plus innovative extensions that exploit the full power of null processing for missing information. FirstSQL is the only RDBMS that has demonstrated a true solution to the EXISTS (subquery) error in ANSI SQL.
While extensions to null facilities like FirstSQL's are essential for robust null processing, an RDBMS must handle nulls without the flaws found in ANSI SQL and most RDBMSs.
The first section desribes the EXISTS or subquery flaw and shows FirstSQL's solution to the flaw. The remaining sections detail the FirstSQL extensions to null processing that exploit the full power of nulls for dealing with missing information.