Keys in Relational Databases - Types and Examples Explained

• 🔐 Introduction to Keys in Relational Databases
• 🧩 Types of Keys in DBMS
  • 1. Primary Key
  • 2. Candidate Key
  • 3. Alternate Key
  • 4. Foreign Key
  • 5. Composite Key
  • 6. Super Key
• 🧾 Summary Table of Key Types
• ✅ Final Thoughts

🔐 Introduction to Keys in Relational Databases

In relational databases, keys are essential for keeping data unique, connected, and consistent. They help uniquely identify records, build relationships between tables, and ensure that data stays accurate and reliable.

This guide will break down all the key types in simple terms—with clear examples.

🧩 Types of Keys in DBMS

1. Primary Key

• A primary key uniquely identifies each row in a table.
• Must be unique and cannot be null.

✅ Example: In a Student table, Roll Number is a primary key because it uniquely identifies each student.

2. Candidate Key

• A candidate key is any field (or group of fields) that can be a primary key.
• A table can have multiple candidate keys, but only one becomes the actual primary key.

✅ Example: In the Student table, both Roll Number and Email (if unique) are candidate keys.

3. Alternate Key

• An alternate key is a candidate key that was not chosen as the primary key.
• It still uniquely identifies records.

✅ Example: If Roll Number is the primary key, then Email is an alternate key.

4. Foreign Key

• A foreign key links one table to another.
• It refers to the primary key in another table and helps enforce referential integrity.

✅ Example: In the Student table, CourseID can be a foreign key that refers to the Course table’s primary key.

5. Composite Key

• A composite key uses two or more attributes together to uniquely identify a row.
• It’s used when no single field is unique on its own.

✅ Example: In an Enrollment table, the combination of StudentID and CourseID can be a composite key.

6. Super Key

• A super key is any combination of attributes that uniquely identifies a row.
• It may include extra attributes not needed for uniqueness.

✅ Example: {Roll Number, Name} is a super key if Roll Number alone is unique.

🧾 Summary Table of Key Types

✅ Final Thoughts

Keys are critical for structuring relational databases correctly. They help enforce:

• Uniqueness
• Relationships
• Data integrity

In the next post, we’ll explore constraints and how they further strengthen relational databases by applying rules on data values.