NULLs
In the real world of information management, data is often missing or incomplete: You forgot to ask for a phone number; a respondent to a questionnaire refuses to divulge his age; a book has been contracted, but the publication date has not been set. Such missing information leaves gaping holes in your set of tidy tables.
The unsightliness of these holes is not the real problem, of course. The danger is that they might introduce inconsistencies into your database. In order to preserve the integrity of your data, the relational model, and Codd's rules, use the concept of nulls to handle missing information.
“NULL” does not mean zero or blank. Rather, it indicates that a value is unknown, unavailable, or inapplicable. Essentially the use ...
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