3.4. Arithmetic and NULLs
NULLs are probably one of the most formidable database concepts for the beginner. Chapter 6 is devoted to a detailed study of how NULLs work in SQL, but this section is concerned with how they act in arithmetic expressions.
The NULL in SQL is only one way of handling missing values. The usual description of NULLs is that they represent currently unknown values that might be replaced later with real values when we know something. This definition actually covers a lot of territory. The Interim Report 75-02-08 to the ANSI X3 (SPARC Study Group 1975) showed 14 different kinds of incomplete data that could appear as the results of operations or as attribute values. They included such things as arithmetic underflow and overflow, ...
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