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CS-502 · Database Management Systems/Quick Revision Short Notes

Database Management Systems (CS-502) - Unit 1 Short Notes

How unit 1 is examined

This unit covers why a DBMS beats file processing, the three-level architecture and DBMS structure, the ER model with generalization, specialization and aggregation, ER-to-table conversion, and the older data models; the marks sit in DBMS structure, generalization, the file-versus-database comparison, data independence, DDL/DML/DCL, ER basics and ER-to-tables.

Database approach v/s Traditional file accessing approach

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Definition. <mark>In the database approach one DBMS stores all data once in a central database and gives every application controlled, shared access to it, whereas in the file approach each application keeps its own private files.</mark>

Key points.

  1. File systems store the same data in many files, so redundancy is high and one update must be repeated everywhere; a DBMS stores it once.
  2. Repeated copies drift apart, causing inconsistency; a DBMS keeps a single copy and so stays consistent.
  3. File data is tied to the program's file format, so changing a record layout means rewriting programs; a DBMS gives data independence.
  4. Files are owned by one application, but a DBMS lets many users share data concurrently.
  5. File programs each code their own integrity rules, but a DBMS enforces them centrally as constraints.
  6. File systems offer only file-level passwords, while a DBMS gives per-user security plus backup and recovery.
Basis File system DBMS
Redundancy High, duplicate files Controlled, single copy
Consistency Frequent inconsistency Maintained by constraints
Data independence None, program depends on layout Physical and logical
Sharing Poor, one application per file Concurrent multi-user
Integrity Coded in each program Enforced centrally
Security File-level only User and object privileges

File operations. Create (new file), open (load its descriptor), read, write, seek (move to a position), close (release), delete, plus rename, append, and directory and access-control operations such as list and set permissions.

Answer frame. Open with both definitions; draw the table; close with "hence the database approach removes redundancy and gives sharing, integrity and security".

Asked: [7 marks] (Nov 2019, Dec 2025) Differentiate database approach and traditional file accessing approach; discuss advantages of database systems. Asked: [7 marks] (Dec 2020) List out the operations that can be performed on files.

Advantages of database systems

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. A DBMS is software that defines, stores, manipulates and controls access to a shared collection of related data.

Key points.

  1. Redundancy is reduced and consistency improves because data is stored once.
  2. Data is shared by many users concurrently, with integrity constraints, security and backup and recovery handled centrally.
  3. Data independence and SQL give easy change, querying and reporting.
  4. For the jewellery artist, one DBMS replaces the word-processor lists and spreadsheets: one customer table serves catalogs and billing, and bills, income and expenses are queried together.

Asked: [7 marks] (Nov 2022) A woman artist keeps mailing list, supplier and customer names and bills in word processor and spreadsheet files; what advantages would a DBMS give her?

Data models

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. A data model is a collection of concepts for describing the structure of data, the constraints on it and the operations on it.

Key points.

  1. High-level (conceptual) models such as ER are close to how users see the world.
  2. Representational (implementation) models such as relational, network and hierarchical sit in between and are used by commercial DBMSs.
  3. Low-level (physical) models describe storage in files and indexes.

Schemas and instances

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. The schema is the overall design of the database and changes rarely; an instance (state) is the data actually in the database at one moment and changes constantly.

Key points.

  1. There are three schemas: physical (internal), logical (conceptual) and view (external).
  2. A schema is like a class declaration and an instance is like the objects created from it.

Data independence

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Definition. <mark>Data independence is the ability to change the schema at one level of the database without having to change the schema at the next higher level.</mark>

Key points.

  1. Physical data independence means changing the internal schema (file organisation, indexes, storage device) without changing the conceptual schema or application programs.
  2. Logical data independence means changing the conceptual schema (adding or dropping a field or table) without changing external views or application programs.
  3. It is essential because programs are insulated from storage changes, so maintenance cost falls, data can migrate to new hardware, and users work at a high level of abstraction.
Basis Physical level Logical level
Describes How data is stored: files, blocks, indexes What data is stored: tables, columns, relationships
Abstraction Lowest, most detail Middle, hides storage detail
Used by DBA and system programmers DBA and database designers
Changes Index, file structure, device Add or remove table or attribute
Independence Physical independence protects the logical level Logical independence protects the view level

Answer frame. Open with the definition; give the two types with one example each; use the table for physical versus logical level; close with why it is essential. For "data models, schemas, instances", first define each from the topics above.

Asked: [7 marks] (Dec 2020) Differentiate physical level and logical level of data abstraction. Asked: [7 marks] (Jun 2020) What is data independence? Why is it essential? Asked: [7 marks] (Dec 2025) Explain data models, schemas and instances. What is data independence?

Database language and interfaces

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Definition. A DBMS provides languages to define the schema (DDL), manipulate data (DML) and control access (DCL), plus interfaces through which users use them.

Key points.

  1. DDL (Data Definition Language) defines the schema, using CREATE, ALTER, DROP, TRUNCATE and RENAME, and its output is stored in the data dictionary.
  2. DML (Data Manipulation Language) retrieves and changes data, using SELECT, INSERT, UPDATE and DELETE; it can be procedural or declarative like SQL.
  3. DCL (Data Control Language) controls privileges, using GRANT and REVOKE.
  4. Interfaces include menu, form, graphical, natural-language, command-line SQL and application program interfaces.
Basis DDL DML DCL
Purpose Define structure Manipulate data Control access
Commands CREATE, ALTER, DROP SELECT, INSERT, UPDATE, DELETE GRANT, REVOKE
Acts on Schema, tables Rows User privileges
Effect Changes the data dictionary, usually auto-commit Changes stored data, can be rolled back Changes permissions

Role in representing the real world (Nov 2023). DDL declares tables, keys and constraints that mirror real entities, and DML stores and updates the real facts in them. The buffer manager brings disk pages into main memory and caches them, so the stored facts are served fast. The data model gives the structure, constraints and operations by which the real world is abstracted into the database.

Answer frame. Open with one line each for DDL, DML, DCL; draw the table; close with "together they define, populate and protect the database".

Asked: [7 marks] (Dec 2020) Differentiate DML, DDL and DCL in detail. Asked: [7 marks] (Nov 2023) Role of (i) DDL and DML, (ii) the buffer manager, (iii) the data model in representing real-world information.

Overall Database Structure

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Definition. <mark>The DBMS structure is a layered system in which users' queries pass through the query processor and storage manager to reach data on disk, while the three-level architecture separates external, conceptual and internal views.</mark>

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u1-01" viewBox="0 0 345 338" width="345" height="338" role="img" aria-label="DBMS structure. 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Usr users, App application programs, QP query processor, TM transaction manager, BM buffer manager, SM storage manager, Disk data files and dictionary.</figcaption></figure> <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u1-02" viewBox="0 0 209 424" width="209" height="424" role="img" aria-label="Three-level architecture. E1 and E2 external views, Con conceptual schema, Int internal schema, DB the stored database."><style>#dsfig-u1-02 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u1-02 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u1-02 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u1-02 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u1-02 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u1-02 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u1-02 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u1-02 .t{fill:#16181D;font-weight:500}#dsfig-u1-02 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u1-02 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u1-02 .dot{fill:#16181D}#dsfig-u1-02 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u1-02 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u1-02 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u1-02 .ah{fill:#454C5A}#dsfig-u1-02 .ah.hi{fill:#2340B8}#dsfig-u1-02 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u1-02 .wl .t{font-size:12px;font-weight:700}#dsfig-u1-02 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u1-02 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u1-02 .e{stroke:#B1B7C3}html.dark #dsfig-u1-02 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u1-02 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u1-02 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u1-02 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u1-02 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u1-02 .t{fill:#E6E8ED}html.dark #dsfig-u1-02 .t.inv{fill:#0F1115}html.dark #dsfig-u1-02 .kd{stroke:#E6E8ED}html.dark #dsfig-u1-02 .dot{fill:#E6E8ED}html.dark #dsfig-u1-02 .ann{fill:#8FA3FF}html.dark #dsfig-u1-02 .lbl{fill:#858D9C}html.dark #dsfig-u1-02 .ptr{fill:#8FA3FF}html.dark #dsfig-u1-02 .ah{fill:#B1B7C3}html.dark #dsfig-u1-02 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u1-02 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u1-02 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u1-02 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah2" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh2" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M48.5,57 L95.1,150.2" marker-end="url(#ah2)"/><path class="e" d="M160.5,57 L113.9,150.2" marker-end="url(#ah2)"/><path class="e" d="M104.5,188 L104.5,277" marker-end="url(#ah2)"/><path class="e" d="M104.5,317 L104.5,363" marker-end="url(#ah2)"/><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">E1</text><circle class="n" cx="169" cy="40" r="18"/><text class="t" x="169" y="40" dy=".35em" text-anchor="middle">E2</text><circle class="n" cx="104.5" cy="169" r="18"/><text class="t" x="104.5" y="169" dy=".35em" text-anchor="middle">Con</text><circle class="n" cx="104.5" cy="298" r="18"/><text class="t" x="104.5" y="298" dy=".35em" text-anchor="middle">Int</text><circle class="n" cx="104.5" cy="384" r="18"/><text class="t" x="104.5" y="384" dy=".35em" text-anchor="middle">DB</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Three-level architecture. E1 and E2 external views, Con conceptual schema, Int internal schema, DB the stored database.</figcaption></figure>

Key points.

  1. The external (view) level shows each user group only the part of the database it needs.
  2. The conceptual (logical) level describes the whole database as entities, attributes and constraints.
  3. The internal (physical) level describes files, indexes and record layout.
  4. Mappings between levels give data independence.
  5. The query processor holds the DDL interpreter, DML compiler and query optimiser, which turn queries into low-level plans.
  6. The transaction manager gives atomic, consistent, isolated and durable execution through concurrency control and recovery.
  7. The storage manager, with its buffer, file and authorisation managers, talks to the OS file system and stores data, indexes and the dictionary.

OS upgrade question. Only the lowest layer, the disk or file manager of the storage manager that calls the OS file operations, must be rewritten to use a new force-to-disk primitive; the query processor and upper layers stay unchanged because of data independence.

For "physical versus logical level", use the table under Data independence.

Answer frame. Open with "a DBMS is a layered system"; draw the structure block diagram, then the three-level figure; develop points 1-4 for levels and 5-7 for components; close with the OS-upgrade answer or with data independence.

Asked: [7 marks] (Nov 2023, Dec 2024) Describe the structure of a DBMS; if the OS gains new file functions, which layers must be rewritten? Draw and explain the detailed system architecture of a DBMS. Asked: [7 marks] (Jun 2020) Differentiate physical level and logical level of data abstraction. Asked: [7 marks] (Jun 2020) Short note on any three: three-level architecture of DBMS, database recovery, triggers in SQL, cursor in SQL, functional dependency.

Functions of DBA and designer

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. The database administrator (DBA) is the person or team with central control over the database and the DBMS.

Key points.

  1. The DBA defines the schema, storage structure and access methods, and modifies them as needs change.
  2. The DBA grants access rights and enforces security and integrity constraints.
  3. The DBA schedules backup and recovery and monitors performance, tuning it when needed.
  4. Database designers decide the data to store, its structures and the views, and coordinate with users to capture their requirements.

Asked: [7 marks] (Dec 2020) Discuss the role of database administrator.

ER data model: Entities and attributes

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Definition. <mark>The ER model describes data as entities, their attributes and the relationships among them, drawn as an ER diagram.</mark>

Key points.

  1. An entity is a distinguishable real-world object, such as a student; a weak entity has no key of its own and depends on an owner entity through an identifying relationship.
  2. An attribute is a property of an entity; it may be simple or composite, single-valued or multivalued, stored or derived, and its domain is its set of allowed values.
  3. A relationship is an association between entities, such as Student enrols in Course.
  4. Mapping cardinality is one-to-one, one-to-many, many-to-one or many-to-many.
  5. Participation is total (every entity takes part, drawn as a double line) or partial.
  6. Key constraint: a key attribute uniquely identifies each entity and is underlined; domain constraint restricts an attribute's values to its domain.
  7. Notation: rectangle entity, double rectangle weak entity, ellipse attribute, double ellipse multivalued, diamond relationship.

Answer frame. Open with "the ER model views the world as entities, attributes and relationships"; sketch Student-Enrols-Course; develop points 1-3, then 4-6 for the constraints question; close with the notation.

Asked: [7 marks] (Jun 2020) Explain various constraints used in the E-R model. Asked: [7 marks] (Dec 2025) Describe the ER data model; explain entities, attributes and relationships with examples.

Entity types

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. An entity type defines a collection of entities with the same attributes, and the entity set is the actual collection of entities of that type at a moment.

Key points.

  1. Entities are strong (have a key) or weak (depend on an owner).
  2. Student is an entity type; Ravi and Sita are entities in the entity set.

Defining the E-R diagram

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. An ER diagram is a picture of entities, attributes and relationships with their keys and cardinalities.

Key points.

  1. Identify entities and keys, then relationships, then cardinality and participation.
  2. Child is a weak entity, keyed by name together with the owner's ssn, and depends on Employee through an identifying relationship.
  3. Works_in is many-to-many between Employee and Department; Manages is one-to-many, each department having exactly one manager.

<figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u1-03" viewBox="0 0 345 338" width="345" height="338" role="img" aria-label="Company ER. Emp (ssn, salary, phone), Dept (dno, dname, budget), Wk Works_in, Mgs Manages, Dep identifying relationship, Chld weak entity (name, age). Dept is total in Mgs; Chld is total in Dep."><style>#dsfig-u1-03 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u1-03 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u1-03 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u1-03 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u1-03 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u1-03 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u1-03 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u1-03 .t{fill:#16181D;font-weight:500}#dsfig-u1-03 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u1-03 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u1-03 .dot{fill:#16181D}#dsfig-u1-03 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u1-03 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u1-03 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u1-03 .ah{fill:#454C5A}#dsfig-u1-03 .ah.hi{fill:#2340B8}#dsfig-u1-03 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u1-03 .wl .t{font-size:12px;font-weight:700}#dsfig-u1-03 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u1-03 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u1-03 .e{stroke:#B1B7C3}html.dark #dsfig-u1-03 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u1-03 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u1-03 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u1-03 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u1-03 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u1-03 .t{fill:#E6E8ED}html.dark #dsfig-u1-03 .t.inv{fill:#0F1115}html.dark #dsfig-u1-03 .kd{stroke:#E6E8ED}html.dark #dsfig-u1-03 .dot{fill:#E6E8ED}html.dark #dsfig-u1-03 .ann{fill:#8FA3FF}html.dark #dsfig-u1-03 .lbl{fill:#858D9C}html.dark #dsfig-u1-03 .ptr{fill:#8FA3FF}html.dark #dsfig-u1-03 .ah{fill:#B1B7C3}html.dark #dsfig-u1-03 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u1-03 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u1-03 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u1-03 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah3" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh3" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,40 L150,40"/><path class="e" d="M188,40 L272,40"/><path class="e" d="M40,59 L40,150"/><path class="e" d="M40,188 L40,270" marker-end="url(#ah3)"/><path class="e" d="M57,48.5 L281,160.5"/><path class="e" d="M298,150 L298,68" marker-end="url(#ah3)"/><g class="wl"><rect x="94.9" y="31" width="19.2" height="18" rx="9"/><text class="t" x="104.5" y="40" dy=".35em" text-anchor="middle">N</text></g><g class="wl"><rect x="223.9" y="31" width="19.2" height="18" rx="9"/><text class="t" x="233.5" y="40" dy=".35em" text-anchor="middle">M</text></g><g class="wl"><rect x="30.4" y="95.5" width="19.2" height="18" rx="9"/><text class="t" x="40" y="104.5" dy=".35em" text-anchor="middle">1</text></g><g class="wl"><rect x="159.4" y="95.5" width="19.2" height="18" rx="9"/><text class="t" x="169" y="104.5" dy=".35em" text-anchor="middle">1</text></g><g class="wl"><rect x="288.4" y="95.5" width="19.2" height="18" rx="9"/><text class="t" x="298" y="104.5" dy=".35em" text-anchor="middle">N</text></g><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">Emp</text><circle class="n" cx="169" cy="40" r="18"/><text class="t" x="169" y="40" dy=".35em" text-anchor="middle">Wk</text><rect class="n" x="273" y="25" width="50" height="30" rx="15"/><text class="t" x="298" y="40" dy=".35em" text-anchor="middle">Dept</text><circle class="n" cx="40" cy="169" r="18"/><text class="t" x="40" y="169" dy=".35em" text-anchor="middle">Dep</text><rect class="n" x="15" y="283" width="50" height="30" rx="15"/><text class="t" x="40" y="298" dy=".35em" text-anchor="middle">Chld</text><circle class="n" cx="298" cy="169" r="18"/><text class="t" x="298" y="169" dy=".35em" text-anchor="middle">Mgs</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Company ER. Emp (ssn, salary, phone), Dept (dno, dname, budget), Wk Works_in, Mgs Manages, Dep identifying relationship, Chld weak entity (name, age). Dept is total in Mgs; Chld is total in Dep.</figcaption></figure>

Asked: [7 marks] (Nov 2023) Draw an ER diagram for employees, departments and children of employees.

Concept of Generalization

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Definition. <mark>Generalization is a bottom-up process that combines lower-level entity sets sharing common attributes into a higher-level superclass entity set.</mark>

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u1-04" viewBox="0 0 307 134" width="307" height="134" role="img" aria-label="Generalization hierarchy, drawn with a triangle labelled ISA. Car and Bike are subclasses; Vehicle holds the shared attributes regno, model and price."><style>#dsfig-u1-04 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u1-04 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u1-04 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u1-04 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u1-04 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u1-04 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u1-04 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u1-04 .t{fill:#16181D;font-weight:500}#dsfig-u1-04 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u1-04 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u1-04 .dot{fill:#16181D}#dsfig-u1-04 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u1-04 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u1-04 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u1-04 .ah{fill:#454C5A}#dsfig-u1-04 .ah.hi{fill:#2340B8}#dsfig-u1-04 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u1-04 .wl .t{font-size:12px;font-weight:700}#dsfig-u1-04 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u1-04 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u1-04 .e{stroke:#B1B7C3}html.dark #dsfig-u1-04 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u1-04 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u1-04 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u1-04 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u1-04 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u1-04 .t{fill:#E6E8ED}html.dark #dsfig-u1-04 .t.inv{fill:#0F1115}html.dark #dsfig-u1-04 .kd{stroke:#E6E8ED}html.dark #dsfig-u1-04 .dot{fill:#E6E8ED}html.dark #dsfig-u1-04 .ann{fill:#8FA3FF}html.dark #dsfig-u1-04 .lbl{fill:#858D9C}html.dark #dsfig-u1-04 .ptr{fill:#8FA3FF}html.dark #dsfig-u1-04 .ah{fill:#B1B7C3}html.dark #dsfig-u1-04 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u1-04 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u1-04 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u1-04 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah4" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh4" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><line class="e" x1="141.5" y1="39" x2="56.5" y2="103"/><line class="e" x1="141.5" y1="39" x2="226.5" y2="103"/><rect class="n" x="104" y="24" width="75" height="30" rx="8"/><text class="t" x="141.5" y="39" dy=".35em" text-anchor="middle">Vehicle</text><circle class="n" cx="56.5" cy="103" r="17"/><text class="t" x="56.5" y="103" dy=".35em" text-anchor="middle">Car</text><rect class="n" x="200.5" y="88" width="52" height="30" rx="8"/><text class="t" x="226.5" y="103" dy=".35em" text-anchor="middle">Bike</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Generalization hierarchy, drawn with a triangle labelled ISA. Car and Bike are subclasses; Vehicle holds the shared attributes regno, model and price.</figcaption></figure>

Key points.

  1. Generalization takes entity sets Car and Bike, finds common attributes (regno, model, price) and moves them to a new superclass Vehicle.
  2. Specialization is the reverse: top-down splitting of Vehicle into subclasses with extra attributes.
  3. Attribute inheritance means each subclass automatically receives all attributes of its superclass, and Car adds only its own, such as doors.
  4. Relationship inheritance means a subclass also takes part in every relationship of its superclass, for example Owns with Person.
  5. Inheritance is useful because shared attributes are stored once, so redundancy falls and code and schema are reused.
  6. Inheritance also enforces constraints uniformly, and hierarchies are disjoint or overlapping, total or partial.
  7. Aggregation, the other half of the 2019 and 2024 question, is drawn under the Aggregation topic.

Answer frame. Open with the definition and "bottom-up"; draw Vehicle-Car-Bike with the ISA triangle; develop points 1-5; close with usefulness. For attribute inheritance, spend the answer on points 3-6. For "specialization vs generalization", use the table under Specialization.

Asked: [7 marks] (Nov 2019, Dec 2024) Explain generalization and aggregation with examples; how to represent generalization, specialization and aggregation in ER diagrams? Asked: [14 marks] (Nov 2022) Short note on any two: Oracle Application Express, Specialization vs Generalization, Temporal vs Multimedia Databases, Triggers. Asked: [7 marks] (Dec 2024) Discuss the mechanism of attribute relationship inheritance; how is it useful?

Aggregation

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. Aggregation is an abstraction in which a relationship set is treated as a higher-level entity so that it can take part in another relationship (a whole-part or "has-a" view).

Key points.

  1. It is needed because ER cannot directly relate a relationship to an entity.
  2. Example: Employee works on Project; the Works_on relationship, drawn as a box, is related to Manager through Monitors.

<figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u1-05" viewBox="0 0 338 338" width="338" height="338" role="img" aria-label="Aggregation. Wo Works_on (Emp, Prj) is aggregated as one unit and related to Mgr manager through Mon Monitors."><style>#dsfig-u1-05 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u1-05 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u1-05 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u1-05 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u1-05 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u1-05 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u1-05 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u1-05 .t{fill:#16181D;font-weight:500}#dsfig-u1-05 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u1-05 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u1-05 .dot{fill:#16181D}#dsfig-u1-05 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u1-05 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u1-05 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u1-05 .ah{fill:#454C5A}#dsfig-u1-05 .ah.hi{fill:#2340B8}#dsfig-u1-05 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u1-05 .wl .t{font-size:12px;font-weight:700}#dsfig-u1-05 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u1-05 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u1-05 .e{stroke:#B1B7C3}html.dark #dsfig-u1-05 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u1-05 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u1-05 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u1-05 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u1-05 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u1-05 .t{fill:#E6E8ED}html.dark #dsfig-u1-05 .t.inv{fill:#0F1115}html.dark #dsfig-u1-05 .kd{stroke:#E6E8ED}html.dark #dsfig-u1-05 .dot{fill:#E6E8ED}html.dark #dsfig-u1-05 .ann{fill:#8FA3FF}html.dark #dsfig-u1-05 .lbl{fill:#858D9C}html.dark #dsfig-u1-05 .ptr{fill:#8FA3FF}html.dark #dsfig-u1-05 .ah{fill:#B1B7C3}html.dark #dsfig-u1-05 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u1-05 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u1-05 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u1-05 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah5" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh5" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,40 L150,40"/><path class="e" d="M188,40 L279,40"/><path class="e" d="M169,59 L169,150"/><path class="e" d="M169,188 L169,279"/><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">Emp</text><circle class="n" cx="169" cy="40" r="18"/><text class="t" x="169" y="40" dy=".35em" text-anchor="middle">Wo</text><circle class="n" cx="298" cy="40" r="18"/><text class="t" x="298" y="40" dy=".35em" text-anchor="middle">Prj</text><circle class="n" cx="169" cy="169" r="18"/><text class="t" x="169" y="169" dy=".35em" text-anchor="middle">Mon</text><circle class="n" cx="169" cy="298" r="18"/><text class="t" x="169" y="298" dy=".35em" text-anchor="middle">Mgr</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Aggregation. Wo Works_on (Emp, Prj) is aggregated as one unit and related to Mgr manager through Mon Monitors.</figcaption></figure>

Specialization

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. Specialization is a top-down process of defining subclasses of an entity set that have distinguishing attributes.

Key points.

  1. Employee specialises into Manager and Clerk, each adding its own attributes.
  2. Subclasses inherit all attributes and relationships of the superclass.
  3. Constraints are disjoint or overlapping, and total or partial.
Basis Specialization Generalization
Direction Top-down Bottom-up
Starts from One superclass Several entity sets
Result Subclasses with extra attributes Superclass of shared attributes

Asked: [7 marks] (Dec 2020) Explain specialization and generalization features of ER diagram with example.

Transforming ER diagram into the tables

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Steps.

Step 1: Strong entity becomes a table; its key is the primary key.
Step 2: Weak entity becomes a table with owner's key plus partial key as primary key.
Step 3: Many-to-many relationship becomes a new table with both keys and its own attributes.
Step 4: One-to-many: put the "one" side's key as a foreign key in the "many" side table.
Step 5: One-to-one: put a foreign key on the total side.
Step 6: Multivalued attribute becomes a separate table; composite becomes its parts.

Example (Nov 2023). R1: E1 has total participation (double line) and an arrow to E2, so many E1 map to one E2. R2: E2 has total participation and an arrow to E1, so many E2 map to one E1. Each relationship merges into the many side.

Table Columns
E1 a (PK), b, c (FK to E2, NOT NULL)
E2 c (PK), d, a (FK to E1, NOT NULL)

The foreign key columns are NOT NULL because participation is total. Answer: no separate R1 or R2 table is needed.

Example (library, Dec 2025). Issue is many-to-many between Member and Book, issued by a Librarian.

Book(isbn PK, title)
Member(mid PK, name)
Librarian(lid PK, name)
Issue(mid FK, isbn FK, lid FK, issue_date)  PK(mid, isbn, issue_date)

Answer frame. Draw the ER diagram first (Member-Issue-Book with Librarian issuing), list the rules used, then give the tables with PK and FK marked.

Asked: [7 marks] (Nov 2023) Given E1(a,b), E2(c,d) with relationships R1 and R2, illustrate the table schema while converting the ER diagram to relational tables. Asked: [7 marks] (Dec 2025) Draw an ER diagram for a library system and transform it into relational tables.

Object oriented data model

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. The object-oriented model stores data as objects, each bundling attributes with methods, grouped into classes.

Key points.

  1. It supports inheritance, encapsulation and object identity.
  2. Objects can be complex and can contain other objects.
  3. It suits CAD and multimedia data.

Network data model

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. The network model represents data as records connected by links, forming a graph in which a child can have many parents.

Key points.

  1. Relationships are set types with an owner and member records (CODASYL).
  2. It handles many-to-many links directly and gives fast navigation.
  3. Programmers navigate pointers, so it is complex.

Relational data model

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. The relational model represents data as tables (relations) of rows (tuples) and columns (attributes), linked by keys.

Key points.

  1. Each relation has a primary key, and foreign keys link relations.
  2. It is queried with SQL, based on relational algebra.
  3. It is simple, has high data independence and underlies Oracle and MySQL.

Comparison between the three types of models

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. Data modelling is describing the data, its relationships and constraints in a formal model before implementation; the hierarchical, network and relational models differ in how they link records.

Key points.

  1. The hierarchical model is a tree in which each child has one parent, so it cannot express many-to-many links directly.
  2. The network model is a graph with multiple parents and pointer links.
  3. The relational model uses tables and keys and is the easiest to use.
Basis Hierarchical Network Relational
Structure Tree Graph Tables
Relationships One-to-many Many-to-many via sets Foreign keys
Access Navigate from root Navigate pointers Declarative SQL
Data independence Low Low High
Example IMS IDMS Oracle, MySQL

Asked: [7 marks] (Nov 2019) What do you mean by data modeling? Compare different data models.

Last-minute revision

  • The database approach stores data once and shares it; file processing duplicates data per application.
  • Data independence: physical (storage changes, logical schema untouched) and logical (schema changes, views untouched).
  • Three levels: external, conceptual, internal.
  • DDL is CREATE, ALTER, DROP; DML is SELECT, INSERT, UPDATE, DELETE; DCL is GRANT, REVOKE.
  • DBMS components: query processor, transaction manager, storage manager including the buffer manager.
  • Generalization is bottom-up, specialization is top-down, both use the ISA triangle.
  • Many-to-many relationship gives a new table; one-to-many gives a foreign key on the many side.
  • Relational is tables and keys, network is a graph, hierarchical is a tree.

Memory hooks

  • "ESI": External, conceptual (Schema), Internal, the three levels from top down.
  • "Many makes a table": a many-to-many relationship always becomes its own table.
  • "D-D-C": Define, Do (manipulate), Control for DDL, DML, DCL.

Coverage checklist

  • Database approach v/s Traditional file accessing approach: Q11, Q12.
  • Advantages, of database systems: Q2.
  • Data models: covered.
  • Schemas and instances: covered.
  • Data independence: Q8, Q9, Q10.
  • Data Base Language and interfaces: Q6, Q7.
  • Overall Database Structure: Q17, Q18, Q19.
  • Functions of DBA and designer: Q16.
  • ER data model: Entitles and attributes: Q14, Q15.
  • Entity types: covered.
  • Defining the E-R diagram: Q13.
  • Concept of Generalization: Q1, Q4, Q5.
  • Aggregation: covered.
  • Specialization: Q20.
  • transforming ER diagram into the tables: Q21, Q22.
  • Various other data models object oriented data Model: covered.
  • Network data model: covered.
  • Relational data model: covered.
  • Comparison between the three types of models: Q3.
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