Database Concepts

Revision Summary: Database Concepts (NCERT Class 12)

1. Chapter at a Glance

• File system stores data in separate files but suffers from data redundancy, inconsistency, isolation, dependence and lack of controlled sharing.
• DBMS is software that creates, manages and provides an abstract view of logically related data stored in a single repository.
• Relational data model organises data into tables (relations) linked through common attributes; tables are not independent.
• Every relation must satisfy three properties: unique attribute names, distinct tuples, and atomic values from the same domain.
• Keys (candidate, primary, composite primary, foreign) enforce uniqueness and relationships between relations.
• Database schema is the logical design (structure, constraints, relationships); database instance is the snapshot of data at a given time.
• Meta-data (data dictionary) stores the schema and constraints; queries retrieve or manipulate data.

2. Key Terms and Definitions

• File: Container to store data (text, CSV, etc.) on a storage device; accessed through application programs.
• Data Redundancy: Duplication of the same data in multiple files.
• Data Inconsistency: Same data stored at different places do not match.
• Data Isolation: Related data files have no mapping/link between them.
• Data Dependence: Changing file structure requires changes in all application programs accessing it.
• DBMS (Database Management System): Software to create, store, manage, update and retrieve data from a database.
• Database Schema: Design/skeleton of a database showing table names, columns, data types, constraints and relationships.
• Data Constraint: Restriction/limitation placed on values that can be stored in a column (e.g., NOT NULL, UNIQUE).
• Meta-data / Data Dictionary: Database catalog that stores schema along with constraints (data about data).
• Database Instance: State/snapshot of the database at any given time after data is loaded.
• Query: Request to a database to obtain information in a desired way.
• Data Manipulation: Operations of Insertion, Deletion or Update on a database.
• Database Engine: Underlying component/set of programs in DBMS that creates the database and handles queries.
• Relation (Table): In relational model, a table that stores data for different columns.
• Attribute (Column/Field): Characteristic/parameter for which data are stored in a relation.
• Tuple (Row/Record): Each row of data in a relation representing a related set of values.
• Domain: Set of values from which an attribute can take a value (usually specified by data type).
• Degree: Number of attributes in a relation.
• Cardinality: Number of tuples in a relation.
• Candidate Key: Attribute(s) that can uniquely identify tuples in a relation.
• Primary Key: Candidate key chosen to uniquely identify tuples in a relation.
• Composite Primary Key: Primary key consisting of more than one attribute when no single attribute is unique.
• Foreign Key: Attribute in one relation whose values are derived from the primary key of another relation (used to link tables).
• Alternate Key: Candidate key not chosen as primary key.

3. Syntax and Constructs

No SQL statements, DDL/DML commands, or query syntax are introduced in this chapter (SQL is mentioned only as the language used for querying, to be covered later).

4. Algorithms and Worked Logic

No step-by-step algorithms or procedures (such as sorting, searching or query execution plans) are described. Students must be able to: - Identify which file-system limitation (redundancy, inconsistency, isolation, etc.) is illustrated by a given scenario. - Distinguish between schema and instance, candidate key and primary key, degree and cardinality. - Determine candidate/primary/composite/foreign keys from given relation schemas and data. - Verify whether a given table satisfies the three properties of a relation.

5. Common Errors and Exam Pitfalls

• Confusing redundancy (duplication) with inconsistency (mismatch after duplication).
• Treating every unique column as a primary key without checking for NULLs or choosing among multiple candidate keys.
• Forgetting that composite primary key is required when no single attribute uniquely identifies tuples (e.g., AttendanceDate + RollNumber).
• Mixing up degree (number of columns) with cardinality (number of rows).
• Assuming foreign key cannot be NULL (NCERT states it can be NULL if it is not part of the primary key of the foreign table).
• Stating that relations are independent tables (NCERT emphasises they must be linked via common attributes).
• Writing definitions using non-NCERT wording (e.g., “row” instead of “tuple”, “column” instead of “attribute”) when exact terminology is expected.