Database.3NF, give example

📘 Third Normal Form (3NF)

A table is in 3NF if:

1. ✅ It is in 2NF
2. ✅ It has no transitive dependencies — i.e., non-key columns do not depend on other non-key columns

---

🔍 What is a Transitive Dependency?

A transitive dependency happens when:

A non-key column depends on another non-key column, which in turn depends on the primary key.

In short:
Primary Key → A → B
Then B is transitively dependent on the primary key via A → ❌ violates 3NF.

---

❌ Example: Not in 3NF

Table: Employees

EmpID (PK)	Name	DeptID	DeptName
1	Alice	10	HR
2	Bob	20	Finance
3	Carol	10	HR

• Primary key: EmpID
• DeptName depends on DeptID
• DeptID depends on EmpID

So:

• EmpID → DeptID → DeptName → ❌ transitive dependency
• This violates 3NF

---

✅ Convert to 3NF: Remove Transitive Dependency

Table 1: Employees

EmpID (PK)	Name	DeptID
1	Alice	10
2	Bob	20
3	Carol	10

Table 2: Departments

DeptID (PK)	DeptName
10	HR
20	Finance

• ✅ Now DeptName depends only on DeptID
• ✅ No transitive dependency in Employees
• ✅ The schema is in 3NF

---

🧠 Summary of Normal Forms:

Form	Rule
1NF	Atomic values, no repeating groups
2NF	No partial dependency on part of composite key
3NF	No transitive dependency (non-key columns shouldn’t depend on each other)