Data Models, Schemas, and Instances

Data Models, Schemas, and Instances

• Data abstraction involves adding or removing detail to suit a user's preferred level, highlighting key components.
• Data Models structure a database to determine its abstraction level.

Categories of Data Models

• Conceptual: Uses entities, attributes, and relationships to mirror user perception of data.
• Representational: Easily understood by end users yet aligned with computer storage organization.
• Physical: Focuses on how data is stored. The models incorporate details like record formats, orderings, and access paths on storage media.

Conceptual (High level) Data Models

• Entity: A real world object or concept.
• Attribute: Represents properties that describe entities.
• Relationship: Represents how entities are associated with each other.

Representational Data models

• Includes relational data models, network and hierarchical models, and record-based data models.

Schemas, Instances, and Database State

• Database schema describes the database using constructs.
• The database state shows the current set of instances or state of data at given time.

Three-Schema Architecture

• Internal Level: Describes the database's physical storage structure.
• Conceptual Level: Describes structure for a community of users.
• External Level: Describes the part of the database that a particular group is interested in, and hides the rest.

Data Independence

• Logical Data Independence: The ability to alter the conceptual schema without changing external schemas or application programs.
• Physical Data Independence: The ability to change the internal schema without affecting the conceptual schema.

DBMS Languages

• Data Definition Language (DDL): Identifies schema constructs and stores schema descriptions in the DBMS catalog.
• Data Control Language (DCL): Manages access control and permissions within the database.
• Transaction Control Language (TCL): Handles transactions in the database, ensuring data integrity and consistency.
• Data Manipulation Language (DML):
  • High-level DML is used to specify complex database operations concisely.
  • Low-level DML retrieves individual records or objects and processes them separately.
• SQL (Query Language): Used for interacting with relational databases and includes DDL, DML, DCL and TCL.

Atomic Transactions

• Atomicity (All-or-Nothing): Guarantees that all transaction operations succeed or none occur. If one part fails, the database returns to its original state.
• Consistency: Requires transactions to maintain database integrity and transition between valid states.
• Isolation: Ensures transactions do not interfere with each other, even if they run concurrently.
• Durability: Once a transaction commits successfully, its effects are permanent, even after system failure.
• The ACID principles (Atomicity, Consistency, Isolation, Durability) ensure reliability, with atomicity being fundamental.