Understanding the power of NULL
Some of you might wonder why a section about NULL is included in this book. Remember that it is all about troubleshooting, and trust me, NULL can be a real source of trouble if its pitfalls are not understood properly. NULL is actually a pretty old thing. E. F. Codd (the father of relational databases) mentioned the concept of NULL in a 1975 paper. The idea of NULL is therefore central to the world of relational systems.
However, NULL has not always been widely understood and accepted among SQL developers. Many people have experienced troubles in the face of NULL. In this section, we will shed some light on this mysterious thing and solve the riddle of NULL.
The first and most important thing to know is that NULL does not mean empty string; it means unknown. Not knowing is not the same as empty. For example, if you know that your wallet is empty, then it means you know that there is nothing in it. Not knowing what is in your wallet is a totally different situation...
