Data Models: Types and Classifications

Data Models

• A data model is a collection of concepts describing database structure with data types, relationships, and constraints.
• Data models include operations for specifying retrievals and updates on the database.

Categories of Data Models

• Databases can be categorized based on the concepts used to describe their structure.
• High-level (conceptual) data models provide concepts close to how users perceive data, also called entity-based or object-based models.
• Low-level (physical) data models describe how data is stored.
• Representational (implementation) data models provide easily understood concepts not too far from computer storage organization.

Database Models

• The Entity-Relationship model is a popular high-level conceptual data model.
• Traditional commercial DBMSs use representational or implementation data models, including Network and Hierarchical Models.

The Network Models

• Network models organize entities in a graph, allowing access through multiple paths.

The Hierarchical Models

• Hierarchical models arrange data as an inverted tree.
• Each entity has only one parent but can have several children.
• The top entity in the hierarchy is called the root.

The Relational Data Model

• Relational Data Models organize data in two-dimensional tables called relations.
• The tables or relations are related to each other.

Schema

• It's important to distinguish between the description of the data and the database itself
• A database schema is the "description of a database," specified during its design, remaining relatively constant over time.

Schema Diagram

• A schema diagram displays the structure of each record type.
• Objects in the schema, such as Student or Course, are called schema constructs.

Database Instance

• A database instance is the actual data stored in a database at a particular moment, and it is also called the database state (or occurrence).

The DBMS Three-Schema Architecture

• The Three-Schema Architecture is to support self-describing database systems, provides insulation between programs and data, and supports multiple data views.
• Schemas are defined on three levels: internal, conceptual, and external.

The Internal Level

• The internal level has an internal schema describing the physical storage structure.
• The internal schema uses a physical data model, detailing data storage and access paths.

The Conceptual Level

• The conceptual level has a conceptual schema that describes the structure of the whole database for a community of users.
• Uses a conceptual or implementation data model.

The External Level

• The external (or view) level includes external schemas or user views.
• Each external schema is based on a data model and describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group.
• Usually uses the same data model as the conceptual level.