Database Topic for IT Officer

Data Models, Schemas and Instances

A characteristic of the database approach is that it provides a level of data abstraction, by hiding details of data storage that are not needed by most users.

A data model is a collection of concepts that can be used to describe the structure of a database.  The model provides the necessary means to achieve the abstraction.

The structure of a database is characterized by data types, relationships, and constraints that hold for the data.  Models also include a set of operations for specifying retrievals and updates.

Data models are changing to include concepts to specify the behaviour of the database application.  This allows designers to specify a set of user defined operations that are allowed.

Categories of Data Models

Data models can be categorized in multiple ways.

• High level/conceptual data models – provide concepts close to the way users perceive the data.
• Physical data models – provide concepts that describe the details of how data is stored in the computer.  These concepts are generally meant for the specialist, and not the end user.
• Representational data models – provide concepts that may be understood by the end user but not far removed from the way data is organized.

Conceptual data models use concepts such as entities, attributes and relationships.

• Entity – represents a real world object or concept
• Attribute - represents property of interest that describes an entity, such as name or salary.
• Relationships – among two or more entities, represents an association among two or more entities.

Representational data models are used most frequently in commercial DBMSs.  They include relational data models, and legacy models such as network and hierarchical models.

Physical data models describe how data is stored in files by representing record formats, record orderings and access paths.

Object data models – a group of higher level implementation data models closer to conceptual data models.

Schemas, Instances and Database State

The description of a database is called the database schema.  The schema is specified during database design, and is not expected to change frequently.

Data models have conventions for displaying schemas as diagrams.  A displayed schema is called a schema diagram. 

  

Each object in the schema is called a schema construct.

Schema diagrams display only some aspects of a schema, such as names and some constraints. 

The data in a database may change frequently, every time records are added or updated.  The data in the database at a given moment in time is called the database state or snapshot. 

Database Schema vs Database State

When a database is defined, the schema is specified to the DBMS.  The database state at this point is in the empty state, with no data. 

The initial state of the database is when the database is first populated or loaded with the initial data.  Every time data is added/removed/updated, there is a new database state.

The DBMS is responsible for ensuring every state is a valid state, a state that satisfies the structure and constraints specified in the schema.

The DBMS stores the descriptions of the schema constructs and constraints, called the meta data, in the DBMS catalogue.

The schema is called the intension, and the database state an extension of the schema.

Three Schema Architecture and Data Independence

Remember from the previous chapters, three of the main characteristics of database systems, these are:

1. Insulation of programs and data
2. Support of multiple views
3. Use of a catalogue to store the database description (schema)

The three schema architecture helps to achieve these characteristics. 

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