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How to cope with missing values - NULLS?

From: Peter Koch Larsen <pkl_at_mailme.dk>
Date: 18 Apr 2003 08:17:45 -0700
Message-ID: <61c84197.0304180717.4b080976@posting.google.com>


How to cope with missing values - NULLS? The to me fundamental problem is how to cope with missing values. I agree that You have no need for them in base tables, but what happens in the situation where you have an outer join? If outer joins are to be provided by the DBMS - and I believe they are to useful to be let out - there must be some means of denoting that a field has no value. There are three ways to go:

  1. Use a NULLABLE type. This could be an extension like the sum-type mentioned (i have not read that thread yet so i can not comment). I see no problems with that approach.
  2. Use a DEFAULT value. This could be feasible in some situations, but in many situations it would not. The most obvious example would be if the field in question is boolean. Which default could you possible use?
  3. Use a marker that existed independently of the field. This approach has so many flaws that I believe it to be infeasible. For one thing, what value should we store in that field? If we have ADT's we must be careful as the ADT might have some constraints, that if not set will invalidate the type - a CIRCLE could as an example be required a non-negative radius. Even non-ADT types might have some integrity constraints: think about a floating point number stored in the IEEE format.

My personal conclusion is that we are stuck with NULLS - if not in the SQL sense then at least in the sense that we are to have some kind of sum type. And if we are stuck with such a type, then the only sensible thing is to have it standardised by the DBMS - how else are we going to let the DBMS perform the outer join by itself?

Kind regards
Peter Received on Fri Apr 18 2003 - 10:17:45 CDT

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