What is an index in SQL?

An index in SQL plays a very crucial role in the realm of database management. It helps to optimize the query performance.

In this article, we will explain the fundamentals of indexes in SQL databases, types of indexes, optimization techniques, and best practices.

Introduction to Index in SQL

An index in SQL database is just like the “table of contents” in a book. Indexes allow the database management system to quickly locate specific rows in a table where it is defined on a specific column.

What is an index in SQL?

An index in SQL is a data structure that provides a quick way to look up data from a table based on the values in one or more columns. An index is like a roadmap that helps the database engine to find the desired data on a fast track.

Imagine you have a library with thousands of books, and you want to find a specific book by its title. Instead of searching through each book one by one, you consult the library’s index, which tells you exactly where to find the book you’re looking for. SQL indexes work similarly but for your database tables.

How to Work An Index in SQL?

SQL indexes create a separate data structure that stores a subset of the data from one or more columns in a table. This data structure is optimized for quick lookups and is organized in a way that reduces the time it takes to find specific rows that match a query condition.

Imagine you have a table of customer information with thousands of entries, and you want to find all customers from a particular city. Without an index, the database would have to scan every row in the table, one by one, to identify the matching records. However, with a properly created index, the database can efficiently jump directly to the relevant rows, saving time and resources.

Importance of An Index in SQL

There are various reasons that show the importance of an index in SQL as follows –

1. Faster Data Retrieval: Indexes speed up data retrieval operations, making queries run much faster, especially on large datasets.
2. Improved Query Performance: Indexes help optimize query performance, which is crucial for applications with heavy database usage.
3. Enforcing Data Integrity: Unique indexes ensure that values in the indexed column(s) are unique, preventing duplicates.
4. Facilitating Joins: Indexes on join columns improve the performance of JOIN operations when combining data from multiple tables.

Type of an Index in SQL?

There are mainly 5 types of indexes in SQL as follows –

1. Single-Column Index: Indexes created on a single column, called single-column index.
2. Multi-Column Index (Composite): Indexes created on multiple columns, called composite index or multi-column index.
3. Unique Index: A unique index ensures the uniqueness of values in specific indexed columns.
4. Clustered Index: Clustered index dictates the physical order of data rows in specific indexed columns.
5. Non-clustered Index: Non-clustered index provides a separate structure for indexing data.

What is a Clustered Index in SQL?

A clustered index determines the physical alphabetical order of data in a table. In other words, the rows in the table are stored on disk in the same order as the index defined on the column. Each table can have only one clustered index because the physical order of data rows can only be maintained in one way.

A clustered index automatically creates a column of a table while creating a primary key constraint. It executes faster and helps to retrieve data than the non-clustered index. Clustered indexes require less amount of memory to complete the operation.

What is a Non-Clustered Index in SQL?

A nonclustered index does not affect the physical order of data according to the nonclustered index key. It creates or maintains a separate data structure that includes a copy of the indexed columns along with a pointer to the actual data rows. Each table can have multiple nonclustered indexes.

A non-clustered index is slower than a clustered index in execution to retrieve data. It requires memory more than the clustered index to complete the operation.

Applications of Index in SQL

You will learn here all about the application of index in SQL as follows –

1. Creation of Single-Column Index

In this example, we will create a single-column index named ‘idx_Employee’ on a column named ‘EmployeeID’ in the ‘Employee’ table using the following statement –

Syntax:

CREATE INDEX Index_Name
ON Table_Name ( Column_Name);

Example:

USE [SQLPOST]
CREATE INDEX idx_Employee
ON Employee (EmployeeID);

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘idx_Employee’ is created on ‘ÉmployeeID’ single column of ‘Employee’ table.

2. Creation of Multi-Column Index

If you want to create an index on multiple columns then in this example, we will create a multi-column index named ‘idx_Employee_Name_Des_Dep_Sal’ on multiple columns i.e. ‘EmployeeName’, ‘Designation’, ‘Department’, and ‘Salary’ etc in the ‘Employee’ table using the following statement –

Syntax:

CREATE INDEX Index_Name
ON Table_Name ( column_name1, column_name2, …., column_nameN);

Example:

USE [SQLPOST]
CREATE INDEX idx_Employee_Name_Des_Dep_Sal
ON Employee (EmployeeName,Designation,Department,Salary);

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘idx_Employee_Name_Des_Dep_Sal’ is created on multiple columns i.e. ‘EmployeeName’, ‘Designation’, ‘Department’, and ‘Salary’ etc in the ‘Employee’ table.

3. Creation of Unique Index

Unique index is similar to the primary key constraint and does not allow duplicate values in the column where it is defined. In this example, we will create a unique index named ‘ux_Employee_EmployeeID’ on a single column i.e. ‘EmployeeID’ in the ‘Employee’ table using the following statement –

Syntax:

CREATE UNIQUE INDEX Index_Name
ON Table_Name ( Column_Name);

Example:

USE [SQLPOST]
CREATE UNIQUE INDEX ux_Employee_EmployeeID
ON Employee (EmployeeID);

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the unique and non-clustered index named ‘ux_Employee_EmployeeID’ is created on a single column i.e. ‘EmployeeID’ in the ‘Employee’ table.

4. Creation of Clustered Index

In this example, we will create a clustered index named ‘CX_Employee_EmployeeID’ on a single column i.e. ‘EmployeeID’ in the ‘Employee’ table using the following statement –

Syntax:

CREATE CLUSTERED INDEX Index_Name
ON Table_Name ( Column_Name ASC/DESC);

Example:

USE [SQLPOST]
CREATE CLUSTERED INDEX CX_Employee_EmployeeID
ON Employee (EmployeeID ASC);

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the clustered index named ‘CX_Employee_EmployeeID’ is created on a single column i.e. ‘EmployeeID’ in the ‘Employee’ table.

5. Creation of Non-Clustered Index

In this example, we will create a non-clustered index named ‘NCX_Employee_EmployeeID’ on a single column i.e. ‘EmployeeID’ in the ‘Employee’ table using the following statement –

Syntax:

CREATE CLUSTERED INDEX Index_Name
ON Table_Name ( Column_Name ASC/DESC);

Example:

USE [SQLPOST]
CREATE NONCLUSTERED INDEX NCX_Employee_EmployeeID
ON Employee (EmployeeID ASC);

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘NCX_Employee_EmployeeID’ is created on a single column i.e. ‘EmployeeID’ in the ‘Employee’ table.

6. Renaming of Index

In this example, we will rename an index named ‘idx_Employee_Name_Des_Dep_Sal’ with a new index named ‘IDX_Employee_NDDS’ in the ‘Employee’ table using the following statement –

Syntax:

EXEC sp_rename 'Table_Name.Old_Index_Name', 'New_Index_Name';
--OR
EXEC sp_rename 'Table_Name.Old_Index_Name', 'New_Index_Name', N'INDEX';

Example:

USE [SQLPOST]
EXEC sp_rename 'Employee.idx_Employee_Name_Des_Dep_Sal', 'IDX_Employee_NDDS';

— OR

USE [SQLPOST]
EXEC sp_rename 'Employee.idx_Employee_Name_Des_Dep_Sal', 'IDX_Employee_NDDS', N'INDEX';

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘idx_Employee_Name_Des_Dep_Sal’ is renamed with a new name ‘IDX_Employee_NDDS’ in the ‘Employee’ table.

7. Deletion or Removal of Index

In this example, we will remove/delete/drop an index named ‘IDX_Employee_NDDS’ from the ‘Employee’ table using the following statement –

Syntax:

DROP INDEX Table_Name.Index_Name;

Example:

USE [SQLPOST]
DROP INDEX Employee.IDX_Employee_NDDS;

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘IDX_Employee_NDDS’ is removed/deleted/dropped from the ‘Employee’ table.

8. Reorganize of Index

In this example, we will reorganize an index named ‘IDX_Employee_NDDS’ from the ‘Employee’ table using the following statement –

Syntax:

ALTER INDEX Index_Name
ON Table_Name REORGANIZE;

Example:

USE [SQLPOST]
ALTER INDEX IDX_Employee_NDDS
ON Employee REORGANIZE;

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘IDX_Employee_NDDS’ from the ‘Employee’ table is reorganized now.

9. Rebuild of Index

In this example, we will rebuild an index named ‘IDX_Employee_NDDS’ from the ‘Employee’ table using the following statement –

Syntax:

ALTER INDEX Index_Name
ON Table_Name REBUILD;

Example:

USE [SQLPOST]
ALTER INDEX IDX_Employee_NDDS
ON Employee REBUILD;

Screenshot: Below screenshot for reference –

Result: You can see in the above screenshot that the non-unique and non-clustered index named ‘IDX_Employee_NDDS’ from the ‘Employee’ table is rebuilt now.

What is the Difference Between a Clustered and Non-Clustered Index?

There are six major differences between a clustered and non-clustered index in SQL as follows –

1. A clustered index determines the physical order of data in a table whereas a non-clustered index, on the other hand, does not affect the physical order of data in the table and creates a separate data structure.
2. Each table can have only one clustered index but a table can have multiple non-clustered indexes.
3. In a clustered index, the table itself is organized based on the clustered index key but in a non-clustered index, the table rows are not physically ordered according to the non-clustered index key.
4. In a clustered index, insert, update, and delete operations may be slower as the physical order of data needs to be adjusted but in a non-clustered index, insert, update, and delete operations are generally faster.
5. The indexed columns of a clustered index are often chosen carefully because they determine the physical organization of the table but you can create non-clustered indexes on columns that are frequently used in search conditions to speed up queries without altering the physical storage of the table.
6. It is common for a clustered index to be created on a column with unique values, but it is not a strict requirement whereas non-clustered indexes can be created on both unique and non-unique columns.

Optimization Technique of Index in SQL

Please refer to some optimization techniques of an index in SQL, as follows –

1. Choosing the Right Columns: Selection of the right column to the index can improve the query performance and hence consider the query patterns and choose the columns accordingly.
2. Choosing the Right Indexes: Understand the difference between each type of index and select the right index to create accordingly.
3. Maintenance of Index: It is recommended to regularly update and maintain your indexes to ensure they remain effective to the business requirement. This involves managing fragmentation and rebuilding indexes as needed.

How can I determine if my SQL Index is effective?

You can measure the effectiveness of an SQL index by analyzing query performance. If your queries execute faster with the index, it’s likely effective. Database management tools often provide query optimization features to help you evaluate index performance.

FAQs

Conclusion

An index in SQL is crucial to speed up that database query retrieval. By implementing the right index in the database table, you can improve query performance and enhance the overall user experience. We hope you like this very well.

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