What Are Data Models In DBMS ?

Data models are a collection of conceptual tools for describing data, data relationships, data semantics and data consistency constraints. A data model provides a way to describe the design of a database at the physical, logical, and view levels. There are main three different data model Types and they may further divided into subcategories which are as follows :

1. Object-based Logical Models

1.a.   The E-R Model  (Entity-Relationship)

1.b.  The Object-Oriented Model  

2.Record-based Logical Models

2.a. The Relational Model.

2.b. The Network Model.

2.c. The Hierarchical Mode.

3. Physical Data Model.

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1.  Object-based logical models:

• Describe data at the conceptual and view levels.
• Provide fairly flexible structuring capabilities.
• Allow one to specify data constraints explicitly.
• Over 30 such models, including
• Entity-relationship model.
• Object-oriented model.
• Binary model.
• Semantic data model.
• Infological model.
• Functional data model. 

­1.a. The E-R Model

1. The entity-relationship model is based on a perception of the world as consisting of a collection of basic objects (entities) and relationships among these objects as it's name itself suggests .
• An entity is a distinguishable object that exists.
• Each entity has associated with it a set of attributes describing it.
• E.g. number and balance for an account entity.
• A relationship is an association among several entities.
• e.g. A cust_acct relationship associates a customer with each account he or she has.
• The set of all entities or relationships of the same type is called the entity set or relationship set.
• Another essential element of the E-R diagram is the mapping cardinalities, which express the number of entities to which another entity can be associated via a relationship set.

We'll see later how well this model works to describe real world situations.

2. The overall logical structure of a database can be expressed graphically by an E-R diagram:
• rectangles: represent entity sets.
• ellipses: represent attributes.
• diamonds: represent relationships among entity sets.
• lines: link attributes to entity sets and entity sets to relationships. 

1.b Object-based logical models:

Object-oriented programming (especially in Java, C++, or C#) has become the popular software development methodology. This led to the development of an object-oriented data model that can be seen as extending the E-R model with notions of encapsulation, methods (functions), and object identity. The object relational data model combines features of the object-oriented data model and relational data model.

• Describe data at the conceptual and view levels.
• Provide fairly flexible structuring capabilities.
• Allow one to specify data constraints explicitly.
• Over 30 such models, including
• Entity-relationship model.
• Object-oriented model.
• Binary model.
• Semantic data model.
• Infological model.
• Functional data model.

2. Record-based logical models:

• Also describe data at the conceptual and view levels.
• Unlike object-oriented models, are used to
• Specify overall logical structure of the database, and
• Provide a higher-level description of the implementation.
• Named so because the database is structured in fixed-format records of several types.
• Each record type defines a fixed number of fields, or attributes.
• Each field is usually of a fixed length (this simplifies the implementation).
• Record-based models do not include a mechanism for direct representation of code in the database.
• Separate languages associated with the model are used to express database queries and updates.
• The three most widely-accepted models are the relational, network, and hierarchical.
• This course will concentrate on the relational model.
• The network and hierarchical models are covered in appendices in the text.

2.a The Relational Model.

• Data and relationships are represented by a collection of tables.
• Each table has a number of columns with unique names, e.g. customer, account.

2.b The Network Model.

• Data are represented by collections of records.
• Relationships among data are represented by links.
• Organization is that of an arbitrary graph. 

2.c The Hierarchical Model.

• Similar to the network model.
• Organization of the records is as a collection of trees, rather than arbitrary graphs.

The relational model does not use pointers or links, but relates records by the values they contain. This allows a formal mathematical foundation to be defined. 

3. Physical Data Models

1. Are used to describe data at the lowest level.
2. Very few models, e.g.
• Unifying model.
• Frame memory.