Data Modelling

Degrees of Data Abstraction

In this section we will look at the database design process in terms of specificity. Just as any design starts at a high level and proceeds to an ever-increasing level of detail, so does database design. For example, when building a home, you start with how many bedrooms and bathrooms the home will have, whether it will be on one level or multiple levels, etc. The next step is to get an architect to design the home from a more structured perspective. This level gets more detailed with respect to actual room sizes, how the home will be wired, where the plumbing fixtures will be placed, etc. The last step is to hire a contractor to build the home. That's looking at the design from a high level of abstraction to an increasing level of detail.

The database design is very much like that. It starts with users identifying the business rules; then the database designers and analysts create the database design; and then the database administrator implements the design using a DBMS.

The following subsections summarize the models in order of decreasing level of abstraction.

External models

• Represent the user's view of the database
• Contain multiple different external views
• Are closely related to the real world as perceived by each user

Conceptual models

• Provide flexible data-structuring capabilities
• Present a "community view": the logical structure of the entire database
• Contain data stored in the database
• Show relationships among data including:
  • Constraints
  • Semantic information (e.g., business rules)
  • Security and integrity information
• Consider a database as a collection of entities (objects) of various kinds
• Are the basis for identification and high-level description of main data objects; they avoid details
• Are database independent regardless of the database you will be using

Internal models

The three best-known models of this kind are the relational data model, the network data model and the hierarchical data model. These internal models:

• Consider a database as a collection of fixed-size records
• Are closer to the physical level or file structure
• Are a representation of the database as seen by the DBMS.
• Require the designer to match the conceptual model's characteristics and constraints to those of the selected implementation model
• Involve mapping the entities in the conceptual model to the tables in the relational model

Physical models

• Are the physical representation of the database
• Have the lowest level of abstractions
• Are how the data is stored; they deal with
  • Run-time performance
  • Storage utilization and compression
  • File organization and access methods
  • Data encryption
• Are the physical level – managed by the operating system (OS)
• Provide concepts that describe the details of how data are stored in the computer's memory