Standard Use of 'Z' Instead of Null to Represent Missing Data

Standard use of 'Z' instead of NULL to represent missing data?

Sack your contractor.

Okay, seriously, this isn't standard practice. This can be seen simply because all RDBMS that I have ever worked with implement NULL, logic for NULL, take account of NULL in foreign keys, have different behaviour for NULL in COUNT, etc, etc.

I would actually contend that using 'Z' or any other place holder is worse. You still require code to check for 'Z'. But you also need to document that 'Z' doesn't mean 'Z', it means something else. And you have to ensure that such documentation is read. And then what happens if 'Z' ever becomes a valid piece of data? (Such as a field for an initial?)

At a basic level, even without debating the validity of NULL vs 'Z', I would insist that the contractor conforms to standard practices that exist within your company, not his. Instituting his standard practice in an environment with an alternative standard practice will cause confusion, maintenance overheads, mis-understanding, and in the end increased costs and mistakes.

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EDIT

There are cases where using an alternative to NULL is valid in my opinion. But only where doing so reduces code, rather than creating special cases which require accounting for.

I've used that for date bound data, for example. If data is valid between a start-date and an end-date, code can be simplified by not having NULL values. Instead a NULL start-date could be replaced with '01 Jan 1900' and a NULL end-date could be replaced with '31 Dec 2079'.

This still can change behaviour from what may be expected, and so should be used with care:

• WHERE end-date IS NULL no longer give data that is still valid
• You just created your own millennium bug
• etc.

This is equivalent to reforming abstractions such that all properties can always have valid values. It is markedly different from implicitly encoding specific meaning into arbitrarily chosen values.

Still, sack the contractor.

How can I avoid NULLs in my database, while also representing missing data?

Good on you, for eliminating Nulls. I have never allowed Nulls in any of my databases.

Of course, if nulls are prohibited, then missing information will have to be handled by some other means. Unfortunately, those other means are much too complex to be discussed in detail here.

Actually it is not so hard at all. There are three alternatives.

1. Here's a paper on How To Handle Missing
   Information Without Using NULL by H Darwen, that may help to get your head around the problem.

   1.1. Sixth Normal Form is the answer. But you do not have to normalise your entire database to 6NF. For each column that is optional, you need a child table off the main table, with just the PK, which is also the FK, because it is a 1::0-1 relation. Other than the PK, the only column is the optional column.

   Look at this Data Model; AssetSerial on page 4 is a classic case: not allAssets have SerialNumbers; but when they do, I want them to store them; more important I want to ensure that they are Unique.

   (For the OO people out there, incidentally, that is a three level class diagram in Relational notation, a "Concrete Table Inheritance", no big deal, we've had it fro 30 years.)

   1.2. For each such table, use a View to provide the 5NF form of the table. Sure, use Null (or any value that is appropriate for the column) to identify the absence of the column for any row. But do not update via the view.

   1.3 Do not use straight joins to grab the 6NF column. Do not use outer joins, either (and have the server fill in a Null for the missing rows). Use a subquery to populate the column, and specify the value that you want returned for a missing value (except if you have Oracle, because its Subquery processing is even worse than its set processing). Eg. and just an eg. you can convert a numeric column to string, and use "Missing" for the missing rows.

When you do not want to go that far (6NF), you have two more options.

2. You can use Null substitutes. I use CHAR(0) for character colomns and 0 for numeric. But I do not allow that for FKs. Obviously you need a value that is outside the normal range of data. This does not allow Three Valued Logic.

3. In addition to (2), for each Nullable column, you need a boolean Indicator. For the example of the Sex column, the Indicator would be something like SexIsMissing or SexLess (sorry). This allows very tight Three Valued Logic. Many people in that 5% like it because the db remains at 5NF (and less tables); the columns with missing info are loaded with values that are never used; they are only used if the Indicator is false. If you have an enterprise db, you can wrap that in a Function, and always use the UDF, not the raw column.

Of course, in all cases, you can never get away from writing code that is required to handle the missing info. Whether it is ISNULL(), or a subquery for the 6NF column, or an Indicator to check before using the value, or an UDF.

If Null has a specific meaning ... then it is not a Null! By definition, Null is the Unknown Value.

Is null harmful?

They are lazy and do not understand the basic idea of a null in the database. Nulls are useful and have a purpose. They should neither be avoided nor used inappropriately.

Is using Null to represent a Value Bad Practice?

I don't think NULL is the best way to do it but you might use a separate tinyInt column to indicate that the row in MyRelatedTable is related to everything in MyTable, e.g. MyRelatedTable.RelatedAll. That would make it more explicit for other that have to maintain it. Then you could do some sort of Union query e.g.

SELECT M.ID, R.ID AS RelatedTableID,....
FROM MyTable M INNER JOIN MyRelated Table R ON R.myTableId = M.Id
UNION
SELECT M.ID, R.ID AS RelatedTableID,....
FROM MyTable M, MyRelatedTable R
WHERE R.RelatedAll = 1

Why should I avoid NULL values in a SQL database?

The NULL question is not simple... Every professional has a personal opinion about it.

Relational theory Two-Valued Logic (2VL: TRUE and FALSE) rejects NULL, and Chris Date is one of the most enemies of NULLs. But Ted Codd, instead, accepted Three-Valued Logic too (TRUE, FALSE and UNKNOWN).

Just a few things to note for Oracle:

1. Single column B*Tree Indexes don't contain NULL entries. So the Optimizer can't use an Index if you code "WHERE XXX IS NULL".

2. Oracle considers a NULL the same as an empty string, so:

   WHERE SOME_FIELD = NULL
   

   is the same as:

   WHERE SOME_FIELD = ''
   

Moreover, with NULLs you must pay attention in your queries, because every compare with NULL returns NULL.
And, sometimes, NULLs are insidious. Think for a moment to a WHERE condition like the following:

WHERE SOME_FIELD NOT IN (SELECT C FROM SOME_TABLE)

If the subquery returns one or more NULLs, you get the empty recordset!

These are the very first few cases that I want to talk about. But we can speak about NULLs for a lot of time...

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