6.2. Missing Values in Columns
The usual description of NULLs is that they represent currently unknown values that may be replaced later with real values when we know something. Actually, the NULL covers a lot of territory, since it is the only way of showing any missing values. Going back to basics for a moment, we can define a row in a database as an entity, which has one or more attributes (columns), each of which is drawn from some domain. Let us use the notation E(A) = V to represent the idea that an entity, E, has an attribute, A, which has a value, V. For example, I could write “John(hair) = black” to say that John has black hair.
SQL’s general-purpose NULLs do not quite fit this model. If you have defined a domain for hair color and ...
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