Data Models: Structure, Operations, and Categories

Data Models

• A data model is a set of concepts that describes the structure of a database to achieve data abstraction.
• Data models include basic operations for specifying retrievals and updates.
• Basic model operations include insert, delete, and update.
• User-defined operations include compute_student_gpa and update_inventory.
• The database designer specifies the data storage, access, and manipulation rules and operations through rules.

Categories of Data Models

• Data models are classified based on the type of concepts used to describe the database structure.

High-Level or Conceptual Data Models

• Focuses on providing concepts that align with users' perception of data.
• Utilizes concepts like entities, attributes, and relationships
• An entity represents a real-world object.
• An attribute further describes an entity.
• A relationship shows associations between two or more entities.
• Can be called entity-based or object-based data models.

Low-Level or Physical Data Models

• Details describe how data is stored on computer storage media with information like access paths
• An access path is a structure that makes the search for particular database records efficient

Implementation (Representational) Data Models

• Represents data using record structure; called record-based models.
• Implementation data models bridge the gap between conceptual and physical models.
• Used in commercial Database Management System (DBMS) implementations like relational, network, and hierarchical data models.

Terminologies

• Database Schema: A description of a database, acting as a blueprint with tables, fields, relationships, and rules.
• Schema Diagram: A display of a database schema.
• Schema Construct: Objects within the schema, such as STUDENT or COURSE.

Database State

• Database State: The data stored in a database at a specific point in time, also called an instance or snapshot.
• Empty Database State: The database schema is specified to the DBMS when defining a new database.
• Initial Database State: Refers to when the database is initially loaded with data.
• Valid State: A state that satisfies the structure and constraints of the database.

Three-Schema Architecture

• The goal is to separate user applications from the physical database.
• The architecture has three levels: internal, conceptual, and external.

Internal Level

• This level describes the physical storage structures and access paths.
• Uses a physical data model.

Conceptual Level

• This level describes the structure and constraints for the whole database.
• Uses a conceptual or implementation data model.

External Level

• Describes various user views.
• Usually uses the same data model as the conceptual schema.
• Each external schema describes the part of the database that a specific user group is interested in and hides the rest.
• Mappings among schema levels are needed to transform requests and data

Schema Level Mappings

• Programs refer to an external schema, and are mapped to the internal schema for execution.
• Data extracted from the internal DBMS level is reformatted to match the user's external view.

Data Independence

• Upper levels are unaffected by changes to lower levels.
• There are two types: logical and physical data independence.

Logical Data Independence

• The ability to change the conceptual schema without changing the external schemas and programs.
• This involves mapping between external and conceptual schemas.

Physical Data Independence

• The ability to change the internal schema without changing the conceptual schema.
• This involves mapping between the conceptual and internal schemas

Schema Changes

• When a schema at a lower level is changed, the mappings between this schema and higher-level schemas need to be changed.
• Higher-level schemas remain unchanged.
• Application programs need not be changed since they refer to the external schemas.

DBMS (Database Management Systems) Languages

• Data Definition Language (DDL): Used by DBAs and designers to specify the conceptual schema of the database.
• DDL defines internal and external schemas (views) in many DBMS systems.
• Some DBMS systems use separate storage definition language (SDL) and view definition language (VDL) to define internal and external schemas.
• Data Manipulation Language (DML): Specifies database retrievals and updates.
• DML commands (data sublanguage) are embedded in a general-purpose programming language (host language) like COBOL, C, C++, or Java.
• A library of functions can access the DBMS from a programming language
• Stand-alone DML commands can be applied directly, called a query language.

Types of DML (Data Manipulation Language)

• High Level or Non-procedural Language

• Allows users to state what data needed rather than how to retrieve it (e.g., SQL).

• Also called declarative languages.

• Also called "set-at-a-time" DMLs, can retrieve many records using a single DML statement.

• Low Level or Procedural Language

• Allows users to tell the system exactly how to manipulate data.

• Embedded in a general-purpose programming language (e.g., C++).

• Also called "record-at-a-time".

Database System Utilities

• Database utilities help DBAs manage the database system.
• Examples include loading, backup, and performance monitoring.
• Loading: Used to load existing data files into a database and includes data conversion tools.
• Backup: Creates a backup copy of the database on a mass storage medium, used to restore the database in failure.
• Performance monitoring: Monitors database usage and provides statistics to the DBA.

Classification of DBMSs

• There are several criteria used to classify DBMSs.

Based on the Data Model

• Relational data model: Represents databases as a collection of tables.
• Object-oriented data model: Defines databases in terms of objects, properties, and relationships.
• Hierarchical data model: Represents data as a tree structure where each child has only one parent.
• Network data model: Represents objects and their relationships in a graph and a child can have more than one parent.

Based on the Number of Users

• Single-user systems: Support only one user at a time.
• Multiuser systems: Support multiple users at the same time and are the most common type of DBMS.