Data Model : An Abstract Model

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What is data model:
Data model is defined in different ways; Here I discuss couple of definitions.
A Data model is an abstract model that organizes elements of data and standardizes how they relate to one another and to properties of the real world. (wekipedia,2016)
A Data model says what information is to be contained in a database, how the information will be used, and how the items in the database will be related to each other. (Dictionary.com)
Data modeling is the process of documenting a complex software system design as an easily understood diagram, using text and symbols to represent the way data needs to flow. (Margaret Rouse)
Data model is managing a large quantity of organized or unorganized data. Data model identify clearly in data modeling.
Types of Data model:
• The conceptual Model
• The Logical Model
• The physical Model
How are data models layered and why?
In database management layers of data is most important, With the help of layers of data we design database.
External layer or External level:
External layer is prior planning level, in external layer we gathered views from users. External provides strong and well maintained security mechanism by hiding parts of the database from certain users. External layer allows users to access data, which design according to their needs, in this way different users access the same data at same time in different way.
Logical layer:
The logical layer is the planning part of the database. In this logical layer we write
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