2.2: Data Abstraction

Data modelling is the first step in the process of database design. This step is sometimes considered to be a high-level and abstract design phase, also referred to as conceptual design. The aim of this phase is to describe:

• The data contained in the database (e.g., entities: students, lecturers, courses, subjects)
• The relationships between data items (e.g., students are supervised by lecturers; lecturers teach courses)
• The constraints on data (e.g., student number has exactly eight digits; a subject has four or six units of credit only)

In the second step, the data items, the relationships and the constraints are all expressed using the concepts provided by the high-level data model. Because these concmepts do not include the implementation details, the result of the data modelling process is a (semi) formal representation of the database structure. This result is quite easy to understand so it is used as reference to make sure that all the user’s requirements are met.

The third step is database design. During this step, we might have two sub-steps: one called database logical design, which defines a database in a data model of a specific DBMS, and another called database physical design, which defines the internal database storage structure, file organization or indexing techniques. These two sub-steps are database implementation and operations/user interfaces building steps.

In the database design phases, data are represented using a certain data model. The data model is a collection of concepts or notations for describing data, data relationships, data semantics and data constraints. Most data models also include a set of basic operations for manipulating data in the database.

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