There are a few final remarks I want to make regarding nulls and 3VL specifically. Nulls and 3VL are supposed to be a solution to the “missing information” problem—but I believe I’ve shown that, to the extent they can be considered a “solution” at all, they’re a disastrously bad one. Before I leave the topic, however, I’d like to raise, and respond to, an argument that’s often heard in this connection. That argument goes something like this:
All of those examples you give where nulls lead to wrong answers are very artificial. Real world queries aren’t like that! More generally, most of your criticisms seem very academic and theoretical—I bet you can’t show any real practical situations where nulls have given rise to the kinds of problems you worry about, and I bet you can’t prove such practical situations do occur.
Needless to say, I have several responses to this argument. The first is: How do we know nulls haven’t caused real practical problems, anyway? It seems to me that if some serious real world situation—an oil spill, a collapsed bridge, a wrong medical diagnosis—were found to be due to nulls, there might be valid reasons (nontechnical ones, I mean) why the information would never get out. We’ve all heard stories of embarrassing failures caused by software glitches of other kinds, even in the absence of nulls; in my opinion, nulls can only serve to make such failures more likely.
Second, suppose someone—me, for example—were to go around claiming that some software product or application contained a serious logical error due to nulls. Can you imagine the lawsuits?
Third and most important, I think those of us who criticize nulls don’t need to be defensive, anyway; I think we should stand the counterarguments on their head, as it were. After all, it’s undeniable that nulls can lead to errors in certain cases. So it’s not up to us to prove those “certain cases” might include practical, real world situations; rather, it’s up to those who want to defend nulls to prove they don’t. And I venture to suggest that in practice it would be quite difficult, and very likely impossible, to prove any such thing.
Of course, if nulls are prohibited, then missing information will have to be handled by some other means. Unfortunately, those other means are a little too complex, in general, to be discussed in detail here. The SQL mechanism of (nonnull) default values can be used in simple cases; but for a more comprehensive approach to the problem—including in particular an explanation of how you can still get “don’t know” answers when you want them, even from a database without nulls—I refer you to Appendix C.
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