What is data model? Explain briefly the E-R data model

Subject Database Management System
NU Year Set: 1.(b) Marks: 5 Year: 2012
A data model (or 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 entities.
The main value of carefully constructing an ERD is that it can readily be converted into a database structure.
There are three components in ERD.
  • Entities: Number of tables you need for your database.
  • Attributes: Information such as property, facts you need to describe each table.
  • Relationships: How tables are linked together.
Relationships
Relationships are the associations between the entities. Verbs often describe relationships between entities. We will use Crow's Foot Symbols to represent the relationships. Three types of relationships are discussed in this lab. If you read or hear cardinality ratios, it also refers to types of relationships.


There is a tradition for ER/data models to be built at two or three levels of abstraction. Note that the conceptual-logical-physical hierarchy below is used in other kinds of specification, and is different from the three schema approach to software engineering.
Conceptual data model
This is the highest level ER model in that it contains the least granular detail but establishes the overall scope of what is to be included within the model set. The conceptual ER model normally defines master reference data entities that are commonly used by the organization. Developing an enterprise-wide conceptual ER model is useful to support documenting the data architecture for an organization.
A conceptual ER model may be used as the foundation for one or more logical data models (see below). The purpose of the conceptual ER model is then to establish structural metadata commonality for the master data entities between the set of logical ER models. The conceptual data model may be used to form commonality relationships between ER models as a basis for data model integration.
Logical data model
A logical ER model does not require a conceptual ER model, especially if the scope of the logical ER model includes only the development of a distinct information system. The logical ER model contains more detail than the conceptual ER model. In addition to master data entities, operational and transactional data entities are now defined. The details of each data entity are developed and the relationships between these data entities are established. The logical ER model is however developed independently of the specific database management system into which it can be implemented.
Physical data model
One or more physical ER models may be developed from each logical ER model. The physical ER model is normally developed to be instantiated as a database. Therefore, each physical ER model must contain enough detail to produce a database and each physical ER model is technology dependent since each database management system is somewhat different.
The physical model is normally instantiated in the structural metadata of a database management system as relational database objects such as database tablesdatabase indexes such as unique key indexes, and database constraints such as a foreign key constraint or a commonality constraint. The ER model is also normally used to design modifications to the relational database objects and to maintain the structural metadata of the database.
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