Database Reference
In-Depth Information
4.8.6 Non-keys
A non-key is a field that does not act as a candidate, primary, alternate, or foreign key. Its
sole purpose is to represent a characteristic of the table's subject. Its value is determined by
the primary key. There is no particular designation for a non-key, so it is not marked in the
table structure.
4.9 CONSTRAINTS ON NULL
A null represents a missing or unknown value. A null does not represent a zero or a text
string of one or more blank spaces. The reasons are quite simple.
 A zero can have a very wide variety of meanings. It can represent the state of an account
balance, the number of available product in a stock.
 Although a text string of one or more blank spaces is guaranteed to be meaningless to
most of us.
A zero-length string two consecutive single quotes with no space in between ('') is also an
acceptable value to languages such as SQL.
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