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

Question

Outside of the argument of whether or not NULLs should ever be used: I am responsible for an existing database that uses NULL to mean "missing or never entered" da

Answer 1

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.