SQL Indexes | Meaning, purpose, examples and types. (2023)


SQL indexes are mostly used for data retrieval. When retrieving data from a database, SQL indexes perform the operation significantly faster. If you want to quickly retrieve data from a database, SQL indexes are an option. The concept of indexing data promises better performance for applications and related queries.

Each book contains an index, probably in the last section. You will see that there are lists or words or phrases in the form of a column with page numbers mentioned alongside. This page helps you identify specific words or phrases from the book quickly. The same concept works for SQL indexes. The data present in the SQL table can be easily sorted using the index chart.

SQL Indexes | Meaning, purpose, examples and types. (1)

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Important points

  • SQL indexes are special tables created to facilitate searching for data in a database.
  • SQL indexes speed up the data recovery process.
  • SQL indexes allow users to quickly search for records and data in a huge database.
  • The basic SQL index commands are CREATE INDEX, CREATE UNIQUE INDEX, and DROP INDEX.
  • There are 6 types of SQL indexes: Clustered, Nonclustered, Unique, Filtered, Columnstore and Hash.

What are SQL indexes?

SQL indexes are reference lists for users to make their search process easier and faster. With the help of SQL index table, it becomes much easier to find records and data that users are looking for in the database.

How do SQL indexes work?

To put it simply, SQL indexes help keep a table sorted and separate to improve the efficiency of the search query process.

When looking at a non-indexed table, sorting the data is a complicated process. It will take a long time and it will be extremely difficult. You, the user, must go through each row and column of the table to find the data you want. Therefore, the indexing concept proves to be an efficient option for data retrieval and search procedures.

(Video) SQL Indexes - Definition, Examples, and Tips


Here's an unindexed EMP_Details table:

Company Nameempirical identificationDepartment
Ravi259Data base
hazel369Data base
Joseph345Data base
mohan547Data base

Now let's search for a specific piece of data by pasting the following code:


SELECTEmp-Name,Emp-ID,Abteilung FROMMP_DetailsWHEREEmp-ID = 785

What will it do?

The process searches each row and column of the table before getting the result of the search query. As you can understand, this is a time-consuming process of deriving the necessary data from the table.

If you look at the table above, you may have no idea of ​​the magnitude of this time-consuming process. Tables in a database are longer than expected. For example, suppose there are a million rows in a table. Can you imagine how long it will take to search for specific dates in such a huge list? This is where indexing is effective.

Look at the table above, but the Dept section is indexed:

Company Nameempirical identificationDepartment
Ravi259Data base
hazel369Data base
Joseph345Data base
mohan547Data base

As the table shown above has the department section indexed, searching by FN department would be easier and less time consuming. When the search starts, the database looks for all departments with FN values. Because it is indexed, the search will be interrupted after the end of the list of FN values.

In fact, the table in the database cannot be automatically reorganized every time the query criteria is changed to maximize the query performance, as this would be impossible.

The database creates a data structure as a result of the index.

(Video) How do SQL Indexes Work

A B-tree is probably the type of data structure. Although the B-tree has several advantages, the most important one for your purpose is its sortability. Due to the clear benefits mentioned above, an ordered data structure improves the effectiveness of your search.

How do I create SQL index database statements?

When creating indexes in SQL database, you must use CREATE INDEX statement.

As mentioned in the previous sections, indexes help retrieve data from a database much faster than normal methods. With the help of indexes, searching for data becomes faster and much easier.

Wait, you need to know something else. Indexes are best used only on columns that must be searched frequently. When a table with indexes is updated it takes much more time compared to a regular form table without indexing.

The CREATE INDEX syntax is used to create SQL index database statements.

CREATE INDEX indexnameON tablename (column1, column2, ...);


Consider the EMP_Detalles table we used in the previous section.

CREATE INDEX DeptON EMP_Details(Dept, EmpID);


The different types of indexes in SQL Server are given below:

1. Grouped Indexes

  • Clustered indexes use the fundamental values ​​of rows of data to store the data in tabular form or to make it easier to read.
  • This type of index is mainly needed when large amounts of data need to change in a database.

2. Non-clustered indexes

  • In nonclustered indexes, rows of data do not have a common structure. This structure is self-contained and includes ungrouped key values.
  • Nonclustered indexes make it easy for users to include non-key columns at a higher level.

3. Unique Indices

  • See Unique indexes for unique values ​​for each row in the index key.
  • There are no duplicate items. This indicates that each row in the table is unique.

4. Filtered indices

  • In filtered indexes, you will notice a minimum number of relevant values ​​in a column for query purposes.

5. Column Storage Indexes

  • In columnar storage indexes, you will find that a lot of data is stored in columns.
  • This index helps improve overall query performance when dealing with large amounts of data.

6. Hash-Indizes

  • In hash indexes, you'll notice unique values ​​like an email address or a primary key in the column they were created on.
  • This type of index is popular due to its fast performance.


Here are the following commands for SQL indexes:

1. Create index command

It is used to create index values ​​for a table. Duplicate values ​​can be used here

(Video) SQL Index |¦| Indexes in SQL |¦| Database Index

Example(ver tabela EMP_Details)

BUILD INDEX IDX_EMPON EMP_Details (Departamento);

2. Create unique index command.

It is used to create index values ​​for a table. No duplicate values ​​can be used here

CREATE UNIQUE INDEX index_nameON table_name(coluna1, coluna2, ...);

Example(ver tabela EMP_Details)


3. Fall rate

It is used to remove an index value from the table.

DROP INDEX tablename.indexname;

Example(ver tabela EMP_Details)


Rename and Drop SQL Indexes

When renaming an index, replace the existing index name with the new name you just used to rename it. However, you must ensure that the new name is unique within the view or table.

The command used to rename the index is sp_rename.

EXEC sp_rename index_name, new_index_name, N'INDEX';

To drop or drop indexes from a table, you must use the DROP INDEX command.

DROP INDEX nomedatabela.nomedoíndice;


SQL indexes are portable and designed to make it easier for users to search records and data compared to searching large and cumbersome tables. These indexes work best for large structured SQL databases.

common questions

Q1 What is the meaning of indexes in SQL?

Responder:Queries use indexes to quickly find information in tables. Both views and tables have built-in indexes. Indexes make it easy to find specific data in a table that contains hundreds of columns and rows. Proper indexing makes the search process smoother and faster.

(Video) Indexes in SQL Server | Use Indexes for Faster Search in SQL | SQL Interview Questions

Q2. How can you distinguish between clustered and non-clustered indexes in SQL?

Responder:In SQL you can only have one clustered index on a table. However, there may be multiple nonclustered indexes on the table. In terms of speed, clustered indexes are faster than non-clustered variants. Clustered indexes never take up additional storage space, but nonclustered indexes do.

Q3 When is the best time to create indexes?

Responder:The best time to create indexes in SQL is when the database consists of a large amount of data and values ​​(non-null values). Indexing helps to retrieve data quickly.

featured articles

This article explains everything about SQL indexes. To learn more about related topics, visit the following links:

  1. SQLUnique Alchemy
  2. Operador SQL LIKE
  3. SQL syntax
  4. Min and Max SQL

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