Database Concepts: CBSE Class 12 Computer Science Notes and Questions

What Database Concepts is about

Weightage: This chapter is part of Unit 3, Database Management, which carries 20 of the 70 theory marks together with Structured Query Language (SQL) and Interface of Python with an SQL Database.Source: CBSE Computer Science (083) syllabus 2026-27, distribution of marks, Unit 3 Database Management

Key concepts

Why a database? Limitations of a file system

NCERT section NCERT 8.2, 8.3

When each office keeps its own files (the fee clerk a fee file, the library a member file), problems appear. Data redundancy: the same data (a student's name and address) is stored many times, wasting space. Data inconsistency: when one copy is updated and another is not, the copies disagree. Data isolation: data is scattered in files of different formats, so combining it needs new programs. Data dependence: programs are tied to the file's structure, so changing the structure means changing every program. Controlled sharing is hard: giving different users different access to parts of a file is difficult. A database is an organised collection of related data, and a DBMS (Database Management System, e.g. MySQL, Oracle, PostgreSQL) is the software that creates, stores, updates, retrieves and protects it, so these problems are reduced: data is stored once, shared, and accessed through queries.

Worked example

A student's address is stored in both the fee file and the transport file. She moves house and only the fee file is updated. Name the two problems this shows and how a DBMS avoids them.

Try it first, then show the solutionSolution

1. Storing the address twice is data redundancy.
2. The two files now give different addresses: data inconsistency.
3. In a database the address is stored once (in one table) and other tables refer to the student by a key, so one update is seen everywhere.

Answer: Redundancy and inconsistency; a DBMS stores the data once and links to it through keys.

Relation, attribute, tuple and domain

NCERT section NCERT 8.4, 8.4.1

In the relational data model (proposed by E. F. Codd), data is kept in tables. A relation is a table. An attribute is a column, with a name such as Name or Class. A tuple is a row: one record. The domain of an attribute is the set of values it may take, for example Class from {11A, ..., 12D} or Phone as 10-digit numbers. Properties of a relation: each attribute name is unique within the table; the order of rows and of columns does not matter; no two tuples are identical; each cell holds a single (atomic) value; a missing value is shown as NULL.

Worked example

In the STUDENT table, name (a) one attribute (b) the tuple of roll number 3 (c) a sensible domain for Class (d) the value of Phone for Joy.

Try it first, then show the solutionSolution

1. (a) Any column heading, e.g. Name.
2. (b) The whole row (3, Meena, 12A, 98100 33333, H1).
3. (c) The set of valid class-section codes, such as 11A to 12D.
4. (d) It is unknown, stored as NULL.

Answer: (a) Name (b) (3, Meena, 12A, 98100 33333, H1) (c) valid class codes such as 11A-12D (d) NULL

Degree and cardinality

NCERT section NCERT 8.4

The degree of a relation is its number of attributes (columns). The cardinality is its number of tuples (rows). Adding a column with ALTER TABLE raises the degree by 1; inserting a row raises the cardinality by 1; deleting rows lowers the cardinality; the header row is never counted. Degree changes rarely (the design changes), cardinality changes all the time (data changes).

Worked example

Find the degree and cardinality of STUDENT and HOUSE. What are they for STUDENT after two new students are added and a column Email is added?

Try it first, then show the solutionSolution

1. STUDENT: 5 columns, 4 rows. HOUSE: 3 columns, 3 rows.
2. Adding two rows: cardinality 4 + 2 = 6. Adding Email: degree 5 + 1 = 6.

Answer: STUDENT 5 and 4; HOUSE 3 and 3; afterwards STUDENT has degree 6 and cardinality 6.

Candidate key, primary key and alternate key

NCERT section NCERT 8.5.1-8.5.3

A candidate key is an attribute (or minimal set of attributes) whose values are unique for every tuple and never NULL, so it could identify each row. A table may have several: in STUDENT both RollNo and Phone are unique, but Phone has a NULL (Joy), so only RollNo qualifies. The primary key is the one candidate key chosen by the designer to identify tuples; it must be unique and not NULL. Every other candidate key is an alternate key. When no single column is unique, a combination can be the key (a composite primary key), such as (RollNo, Subject) in a marks table where each student has many subjects.

Worked example

A table BOOK has columns ISBN, AccessionNo, Title, Author. Every copy has a unique AccessionNo; every edition has a unique ISBN, but the library has two copies of some editions. Identify the candidate keys, a primary key and the alternate key(s).

Try it first, then show the solutionSolution

1. ISBN repeats for two copies of the same edition, so it is not unique in this table: not a candidate key.
2. AccessionNo is unique for every row and never empty: the only candidate key.
3. So the primary key is AccessionNo, and there is no alternate key.

Answer: Candidate key: AccessionNo; primary key: AccessionNo; no alternate key (ISBN is not unique here).

Foreign key and referential links

NCERT section NCERT 8.5.4

A foreign key is an attribute in one table that refers to the primary key of another table (or the same table), linking the two. Its values must either match an existing primary-key value in the referenced table or be NULL; this rule is referential integrity. The table holding the foreign key is the referencing (child) table; the other is the referenced (parent) table. A foreign key may repeat (many students in one house) even though the primary key it refers to cannot.

Worked example

In STUDENT and HOUSE, which attribute is the foreign key and in which table? Can a new student be given HouseID H5?

Try it first, then show the solutionSolution

1. HouseID is the primary key of HOUSE.
2. STUDENT.HouseID refers to it, so HouseID in STUDENT is a foreign key.
3. H5 does not exist in HOUSE, so referential integrity forbids it.

Answer: HouseID in STUDENT is the foreign key referring to HOUSE; H5 is not allowed until house H5 is added to HOUSE.

Database Concepts in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Relational Data Model TermsComing soon
• Candidate, Primary and Alternate KeysComing soon
• Foreign KeyComing soon
• Keys: Candidate, Primary, Alternate and Foreign Key
• Need for a Database: Limitations of the File System
• Relational Model: Relation, Attribute, Tuple, Domain, Degree, Cardinality

Database Concepts: syntax and key facts

Relational terms

relation = table; attribute = column; tuple = row; domain = set of allowed values of an attribute

A missing value is NULL, which is not zero or a blank.

Degree and cardinality

degree = number of columns; cardinality = number of rows (header not counted)

ALTER TABLE ... ADD changes degree; INSERT and DELETE change cardinality.

Keys

candidate key: unique and not NULL; primary key: the chosen candidate key; alternate key = candidate keys - primary key

A primary key can be composite (two or more columns together).

Foreign key rule

foreign key value ∈ {primary-key values of the parent table} ∪ {NULL}

This is referential integrity.

File-system problems

redundancy, inconsistency, isolation, dependence, difficult controlled sharing

A DBMS reduces all of them.

How the board asks Database Concepts

The question patterns to expect, and the method that works for each.

Tricks to remember Database Concepts

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Degree across, Cardinality counts the cards

  Degree counts the column headings across the top; Cardinality counts the record Cards (rows) stacked below.

  Use it when: Never swapping degree and cardinality.

• CAP keys

  Candidates apply, one becomes Principal (primary), the rest are Alternates.

  Use it when: Candidate, primary and alternate keys.

• Foreign key = passport

  It is valid only if the home country (parent table) has issued that number.

  Use it when: Referential integrity.

• RIIDS

  Redundancy, Inconsistency, Isolation, Dependence, Sharing problems.

  Use it when: Listing file-system limitations.

Database Concepts: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Database Concepts

The slips examiners see most often, and what to write instead.

• ✗Mistake: Counting the header row in cardinality

  The header holds attribute names, not data.

  Wrong working

  STUDENT has 5 rows including the header, so cardinality 5

  Right working

  STUDENT has 4 tuples, so cardinality 4

  ✓Do this: Count only data rows.

  Usually costs: 1 mark

• ✗Mistake: Choosing a column with NULL or repeated values as the primary key

  Primary keys must be unique and not NULL.

  Wrong working

  Primary key of STUDENT: Phone

  Right working

  Primary key of STUDENT: RollNo (Phone has a NULL for Joy)

  ✓Do this: Scan the column for duplicates and blanks before choosing.

  Usually costs: 1 mark

• ✗Mistake: Saying a foreign key must be unique

  Many child rows can refer to the same parent row (two students in house H1).

  ✓Do this: Foreign-key values may repeat; they must match a parent primary key (or be NULL).

  Usually costs: 1 mark in an assertion-reason item

• ✗Mistake: Calling every unique column the primary key

  There is only one primary key; other candidate keys are alternate keys.

  ✓Do this: List all candidate keys, choose one as primary, and call the rest alternate keys.

  Usually costs: Half to 1 mark

NCERT exercise guide: Database Concepts

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Database Concepts

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   The number of attributes in a relation is called its: (a) cardinality (b) degree (c) domain (d) tuple

   Hint

   Columns.

   Answer

   (b) degree

2. Q2 · 1 mark · Very short answerBoard level

   Define a candidate key and give an example from a table of your choice.

   Hint

   Unique, not NULL.

   Answer

   An attribute or minimal set of attributes that uniquely identifies every tuple and is never NULL, e.g. AdmissionNo or Aadhaar in a STUDENT table.

3. Q3 · 1 mark · Very short answerHarder board level

   A relation R has degree 4 and cardinality 7. A relation S has degree 3 and cardinality 5. (a) What is the total number of attribute values (cells) in R? (b) If two columns are added to S, what is its new degree?

   Hint

   Cells = rows × columns.

   Answer

   (a) 4 × 7 = 28 (b) 3 + 2 = 5

4. Q4 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): The values of a foreign key column must all be different.
   Reason (R): A foreign key refers to the primary key of another table.

   Hint

   Two students can be in the same house.

   Answer

   (d) A is false but R is true.

5. Q5 · 2 marks · Short answerHarder board level

   TEACHER(TID, Name, Subject, Email) and CLASS(ClassID, Section, ClassTeacherTID). Identify the primary key of each table and the foreign key, and state what rule the foreign key enforces.

   Hint

   Which column names a teacher?

   Answer

   TEACHER: TID; CLASS: ClassID. Foreign key: ClassTeacherTID in CLASS referring to TEACHER.TID. It enforces referential integrity: every class teacher must be an existing teacher (or NULL).

6. Q6 · 2 marks · Short answerHOTS / competency

   Why is the order of rows not important in a relation? Give one consequence for queries.

   Hint

   Sets of tuples.

   Answer

   A relation is a set of tuples, identified by key values, not by position. So a query cannot rely on 'the third row'; to get rows in an order you must ask for it (ORDER BY in SQL).

7. Q7 · 2 marks · Short answerBoard level

   Give two advantages of a DBMS over a file system with examples.

   Hint

   Redundancy and sharing.

   Answer

   Reduced redundancy: a student's details are stored once and referred to by RollNo. Controlled sharing and security: the accounts clerk can be given access to fee data but not to marks.

8. Q8 · 2 marks · Short answerHOTS / competency

   In the canteen tables, a sale row (104, NULL, 2) is inserted. Is it allowed by referential integrity? Is it sensible? Explain.

   Hint

   Foreign keys may be NULL; but is ItemCode part of the primary key?

   Answer

   A NULL foreign key is allowed by referential integrity in general, but here ItemCode is part of SALE's composite primary key (BillNo, ItemCode), and a primary key cannot contain NULL, so the row is rejected; it would also be meaningless, as a sale must be of some item.

9. Q9 · 1 mark · Very short answerBoard level

   A relation has the attributes (RegNo, Name, DOB, Mobile, Email). RegNo, Mobile and Email are unique and compulsory. If RegNo is the primary key, list the alternate keys.

   Hint

   Candidates minus primary.

   Answer

   Mobile and Email.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is a tuple?Show answer

   A row of a relation (one record).

2. 2.What is the degree of a relation?Show answer

   The number of attributes (columns).

3. 3.What is the cardinality of a relation?Show answer

   The number of tuples (rows).

4. 4.Define domain.Show answer

   The set of permissible values for an attribute.

5. 5.What is an alternate key?Show answer

   A candidate key that is not chosen as the primary key.

6. 6.Can a primary key have NULL values?Show answer

   No; it must be unique and not NULL.

7. 7.What does a foreign key refer to?Show answer

   The primary key of another table (or of the same table).

8. 8.Name two limitations of a file system.Show answer

   Any two: data redundancy, data inconsistency, data isolation, data dependence, difficulty in controlled sharing.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Database: organised collection of related data; DBMS: software to manage it (MySQL).
• ☐File-system problems: redundancy, inconsistency, isolation, dependence, hard controlled sharing.
• ☐Relation = table; attribute = column; tuple = row; domain = allowed values.
• ☐Degree = number of columns; cardinality = number of rows.
• ☐Candidate key: unique, not NULL. Primary key: the chosen one. Alternate: the rest.
• ☐Foreign key refers to a primary key in another table; values may repeat; must exist in the parent or be NULL.
• ☐Row order and column order do not matter in a relation; no duplicate tuples.

Database Concepts: frequently asked questions