18. Working with Data Types and Data Display Formats : Data Types : Nulls
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Nullability is an attribute of stored data. A null represents inapplicable or missing data. It is most useful when working with numeric data used in aggregate computations such as totals and averages. An example of nullable text is an unfilled Home Address field on a personnel form for a new employee when not all information is known.
You can determine which of your tables' columns are to be nullable. In this way, missing data, default values, or empty strings cannot be assigned a value of zero.
It is important to distinguish between ASCII NULL character and column nullability. The NULL character, ASCII octal \000, is a value permitted in certain character data types. Nullability is a condition or status that is available to all the data types. A user assigns nullability to a specific column in a table while creating the table.