An instance defines the general shape of an object you want to represent in a database. You can use it to create many classes in the same shape

If the  Cat  and  Dog  classes inherit the  Animal  class, you can say that these two classes are instances of  Animal  .

If you store Ms. Smith’s labrador in a database whose conceptual model contains a  Dog  class, this labrador would be one instance of the  Dog  class.

There is data redundancy if none of these attributes are considered derived attributes.

All three of these attributes are derived because you can calculate any of them from the other two.

If none of these attributes are derived attributes, there could be inconsistencies within the database. For example, if a person enters an invoice into the software with a “totalExTax = $1000,” “TaxRate = 5%,” and “totalIncTax = $1000”. When actually, the total including tax is $1050 ($1000 + 5% = 1050).

Because the total including tax is calculated using the total (excluding tax) and the tax rate, it would be best to consider the total including tax to be an attribute derived from the other two.

A whole number greater than or equal to 0 

A whole number

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