Database.Explain 2nf, give example

📗 Second Normal Form (2NF)

2NF (Second Normal Form) builds on 1NF and adds one more rule:

A table is in 2NF if:

1. It is in 1NF, and
2. All non-key attributes are fully functionally dependent on the entire primary key.

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🔍 What Does “Fully Dependent” Mean?

If the primary key is composite (made of two or more columns), then:

• A non-key column must depend on both parts of the key, not just one.

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❌ Example: Not in 2NF

Table: StudentCourses

StudentID	CourseID	StudentName	CourseName
1	CS101	Alice	Java
1	CS102	Alice	SQL
2	CS101	Bob	Java

• Composite Primary Key: (StudentID, CourseID)
• Problem:
  • StudentName depends only on StudentID
  • CourseName depends only on CourseID

❌ These are partial dependencies, so this table is not in 2NF.

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✅ Convert to 2NF: Split into Two Tables

Table: Students

StudentID	StudentName
1	Alice
2	Bob

Table: Courses

CourseID	CourseName
CS101	Java
CS102	SQL

Table: StudentCourses

StudentID	CourseID
1	CS101
1	CS102
2	CS101

• ✅ Now, all non-key attributes fully depend on their entire primary key.
• ✅ This structure is in 2NF.

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💡 Summary:

Normal Form	Key Rule
1NF	Atomic values, no repeating groups
2NF	No partial dependency on a composite key