Index vs Unique index

1. Index

An index is a data structure that improves the speed of data retrieval operations on a table. It acts like a quick reference book, allowing the SQL Server to locate rows quickly without scanning the entire table.

Characteristics:

• Speed up queries: An index helps accelerate SELECT queries by allowing the database to find rows faster.

• Allows duplicates: An index can contain duplicate values.

Example:

CREATE INDEX idx_employee_lastname
ON employees (lastname);

2. Unique Index

A unique index is a special type of index that ensures all the values in the indexed column are unique. This means no two rows in the table can have the same value for the indexed column.

Characteristics:

• Speed up queries: Similar to a regular index, a unique index speeds up queries.

• Ensures uniqueness: A unique index ensures that the values in the column(s) are unique across the table.

• Data integrity: A unique index is often used to enforce unique constraints on columns.

Example:

CREATE UNIQUE INDEX idx_employee_email
ON employees (email);

Comparison:

Feature	Index	Unique Index
Primary Purpose	Speed up queries	Speed up queries and ensure uniqueness
Duplicate Values	Allowed	Not allowed
Primary Use Case	Performance optimization for queries	Data integrity and query optimization

When to Use:

• Index: Use when you need to speed up queries on one or more columns without requiring the values in that column to be unique. For example, searching by product name or product description.

• Unique Index: Use when you need to ensure that no two rows have the same value in a specific column or set of columns. For example, ensuring that email addresses are unique in a users table.

Practical Examples:

Index:

Suppose you have a products table and you frequently search for products by product name. You can create an index on the product_name column to speed up the query.

CREATE INDEX idx_products_name
ON products (product_name);

Unique Index:

Suppose you have a users table and you want to ensure that each user's email address is unique. You can create a unique index on the email column.

CREATE UNIQUE INDEX idx_users_email
ON users (email);

Conclusion:

• Index: Improves query performance but does not enforce uniqueness.

• Unique Index: Improves query performance and ensures that the values in the indexed columns are unique.

Understanding the difference between these two types of indexes will help you design your database more efficiently, optimize queries, and ensure data integrity.