What is Denormalization in SQL? Simple Explanation for Beginners

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What is Denormalization?

  • Definition:
    Denormalization is the process of combining tables or adding redundant data to a database to make read queries faster, even if it means data might be repeated.
  • Why it happens:
    In a normalized database (1NF, 2NF, 3NF), data is stored in separate tables to avoid redundancy.
    But sometimes, joining many tables during queries slows down performance.
    Denormalization helps reduce joins and speed up reading data.
  • Important:
    Denormalization sacrifices storage space and may lead to redundancy but makes querying faster
What is Denormalization in SQL? Simple Explanation for Beginners

Example:

Imagine you have a normalized database with these two tables:

1. Students Table

StudentIDNameClassID
1Rahul101
2Priya102

2. Classes Table

ClassIDClassName
101Maths
102Science

his is normalized (no repeated data).

But if you want to show a list of students with their class name, you need a JOIN query:

sql
SELECT s.Name, c.ClassName
FROM Students s
JOIN Classes c ON s.ClassID = c.ClassID;

Denormalized version:

Instead of two tables, you combine them into one table like this:

StudentIDNameClassIDClassName
1Rahul101Maths
2Priya102Science
  • Now, you don’t need JOINs to get student + class info.
  • Queries are faster, but ClassName is repeated for every student in the same class.

Key Points

  1. Denormalization = less JOIN, more redundancy
  2. Helps read-heavy databases like reporting or analytics.
  3. Use carefully because it can cause data inconsistency if updates aren’t handled properly.

Normalized Database

Students Table
+-----------+-------+---------+
| StudentID | Name  | ClassID |
+-----------+-------+---------+
| 1         | Rahul | 101     |
| 2         | Priya | 102     |
+-----------+-------+---------+

Classes Table
+---------+-----------+
| ClassID | ClassName |
+---------+-----------+
| 101     | Maths     |
| 102     | Science   |
+---------+-----------+
  • No repeated data
  • Need JOIN to get student + class info
sql
SELECT s.Name, c.ClassName
FROM Students s
JOIN Classes c ON s.ClassID = c.ClassID;

Denormalized Database

Students Table (Denormalized)
+-----------+-------+---------+-----------+
| StudentID | Name  | ClassID | ClassName |
+-----------+-------+---------+-----------+
| 1         | Rahul | 101     | Maths     |
| 2         | Priya | 102     | Science   |
+-----------+-------+---------+-----------+
  • No JOIN needed
  • Query faster
  • ClassName repeated → redundancy
sql
SELECT Name, ClassName
FROM Students;

Summary:

  • Normalized = less redundancy, more JOINs
  • Denormalized = more redundancy, fewer JOINs, faster reads