DBMS

🔑 PART 1: KEYS IN DBMS (MOST SCORING)

🔹 What is a Key?

👉 A key is an attribute (or set of attributes) that uniquely identifies a record in a table.

🧠 Memory Trick:

“S C P F A” → Super, Candidate, Primary, Foreign, Alternate

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🔹 Types of Keys (Exam Favorite)

Key	Meaning	Easy Example
Super Key	Can uniquely identify record	RollNo, (RollNo + Name)
Candidate Key	Minimal Super Key	RollNo
Primary Key	Chosen candidate key	RollNo
Alternate Key	Candidate key not chosen	Aadhaar
Foreign Key	Refers to PK of another table	DeptID
Composite Key	More than one attribute	(RollNo + Subject)

📌 MCQ Tip
✔ Every Primary Key is a Candidate Key
❌ Every Candidate Key is not Primary Key

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🔥 One-Line Exam Facts

• Primary key cannot be NULL

• Foreign key can be NULL

• One table → only one Primary Key

• Multiple Candidate Keys possible

📐 PART 2: NORMALIZATION (VERY HIGH WEIGHTAGE)

🔹 What is Normalization?

👉 Process of removing redundancy and avoiding anomalies

🧠 Memory Line:

“N R A” → No Redundancy, Avoid Anomaly

🔹 Types of Anomalies (1 MCQ sure)

Type	Meaning
Insert Anomaly	Cannot insert data
Update Anomaly	Update in many places
Delete Anomaly	Data loss

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🔹 Normal Forms (ULTRA IMPORTANT)

✅ 1NF (First Normal Form)

✔ No multivalued attributes
✔ Atomic values only

❌ {Maths, Science} → Wrong
✔ Maths, Science → Correct

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✅ 2NF

✔ Must be in 1NF
✔ No Partial Dependency

🧠 Trick:

Partial dependency occurs only when PK is composite

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✅ 3NF (MOST ASKED)

✔ Must be in 2NF
✔ No Transitive Dependency

🧠 Trick:

Non-key → Non-key = Transitive (❌)

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🔥 Normalization Summary (MEMORIZE)

NF	Removes
1NF	Multivalued
2NF	Partial dependency
3NF	Transitive dependency

📌 Exam Fact:
👉 3NF is sufficient for most databases

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🧮 PART 3: SQL (GUARANTEED MARKS)

🔹 SQL Categories (MCQ FAVORITE)

Type	Commands
DDL	CREATE, DROP, ALTER
DML	INSERT, UPDATE, DELETE
DQL	SELECT
DCL	GRANT, REVOKE
TCL	COMMIT, ROLLBACK

🧠 Trick:

DDL–DML–DCL–TCL

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🔹 Important SQL Commands

SELECT

SELECT * FROM Student;

WHERE

SELECT * FROM Student WHERE Marks > 60;

GROUP BY

👉 Used with aggregate functions

SELECT Dept, COUNT(*) FROM Student GROUP BY Dept;

HAVING

👉 Works after GROUP BY

SELECT Dept FROM Student GROUP BY Dept HAVING COUNT(*) > 5;

📌 MCQ Trap

• ❌ WHERE with aggregate → Wrong

• ✔ HAVING with aggregate → Correct

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🔹 Aggregate Functions (VERY IMPORTANT)

Function	Meaning
COUNT()	Number
SUM()	Total
AVG()	Average
MAX()	Maximum
MIN()	Minimum

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🔥 SQL JOIN (1–2 MCQ sure)

Join	Meaning
INNER JOIN	Common records
LEFT JOIN	All left + matched
RIGHT JOIN	All right + matched
FULL JOIN	All records

🧠 Trick:

LEFT → Left table full

🧩 PART 4: RELATIONAL ALGEBRA (CONCEPTUAL MCQs)

🔹 What is Relational Algebra?

👉 Procedural query language

👉 Uses operators

🔹 Important Operators (MUST REMEMBER)

Symbol	Operator
σ	Selection
π	Projection
∪	Union
−	Set Difference
×	Cartesian Product
⨝	Join

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🔹 Examples (VERY SIMPLE)

Selection

σ Marks > 60 (Student)

Projection

π Name, Marks (Student)

📌 MCQ Tip

• Selection → Rows

• Projection → Columns

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🎯 FINAL EXAM MEMORY SHEET (REVISE DAILY)

🔑 Keys

✔ Primary = Unique + Not Null
✔ Foreign = Reference

📐 Normalization

✔ 1NF → Atomic
✔ 2NF → No Partial
✔ 3NF → No Transitive

🧮 SQL

✔ WHERE ≠ HAVING
✔ GROUP BY → Aggregate

🧩 Relational Algebra

✔ σ → Rows
✔ π → Columns