What are Indexes in SQL: Complete Guide on SQL Index with Examples

What are Indexes in SQL: There are various advantages of using indexes in SQL.

If you are already familiar with the pointers in programming, then you can easily understand the working of indexes in SQL.

As pointers are used to store the reference of the original Data. Similarly, In SQL, we use indexes to store the reference of Data in the Table.

So, Indexes are not the original Data. Instead, they are used as a copied image of the original Data.

So, if you have a huge amount of data on a Database and you want to retrieve some particular data from it more often. Then you can create an index of that Data and use that Data where ever you want.

So, now that you have a basic understanding of the Indexes in SQL. Let’s understand it in more detail with the help of the definition.

Table of Contents

What is SQL Index

An Index is a reference of the table in a Database and is used for fast retrieval of data and enhances the performance of the Database.

You can understand the working of the Index more easily with the help of the below example.

In this article, you can see the Table of Content, which we have used as a reference to all the heading in this blog post, so you can easily get to know all the topics that we have covered in this article.

However, if we make changes to any of our heading in this article, then the Table of Content will automatically get updated.

Similarly, Index in SQL is the reference to the Data in the Table of the Database. So, you can frequently access the data that you want more often.

Here also if you make any change in the original Data of the Database, then its reference will automatically get updated as well.

So, hope you understand the working of the Index. Don’t worry if you still don’t get it. Because now we will understand Index in more detail with the help of examples and its uses.

CREATE INDEX Command in SQL

The CREATE INDEX Command is used to create an index of the table in SQL.

The basic syntax of using CREATE INDEX Command in SQL is:

				
					CREATE INDEX indexName ON tableName;

Here, indexName is the name of the Index which you want to create and the tableName is the name of the Table for which you are creating an index.

Now, that you understand the basic syntax of creating an Index. Let’s take a look at the variations of this Command, which will enhance the functionality of CREATE INDEX Command.

1. Creating Index for Single Column

We can create an index for a Single Column of a Table by using parenthesis () in the CREATE INDEX command.

The syntax of creating an Index for a Single Column is:

				
					CREATE INDEX indexName ON tableName (columnName);

So, To index only the specific column of a Table, you have to make use of parenthesis () and inside the parenthesis, you have to specify the name of the column that you want to Index.

In the above example, columnName is the name of the column in the table for which you want to create an Index.

Now, let’s see another variation of this command, in which we will create an index for more than one column in SQL.

2. Creating Index for Multiple Column

The basic syntax of creating an Index for Multiple Column is the same as of creating an Index for a Single Column.

However, the only difference is that here, we use a comma as a separator to specify the name of different Columns for which we want to create an Index.

The syntax of creating an Index for Multiple Column is as follow:

				
					CREATE INDEX indexName ON tableName (columnName1, columnName2);

As you can see that we have separated the column name using a comma. So, you can index as many columns as you want using this syntax.

3. Creating Unique Index

If you want to remove the duplicate or similar values from the Index that you are creating. Then you can use the UNIQUE keyword.

While creating an index of any column in a Table, we can remove the duplicate values from it using the UNIQUE command.

The basic syntax of using a UNIQUE Command is:

				
					CREATE UNIQUE INDEX indexName ON tableName (columnName);

As you can see the syntax of this command is very similar to the above-discussed commands.

However, the only difference is that here we have used the UNIQUE keyword, which makes this Index a Unique Index and removes all the duplicate values from it.

DROP INDEX Command in SQL

The DROP INDEX Command is used to delete the Index that we have created.

The syntax for removing the index is:

				
					DROP INDEX indexName;

Here, indexName is the name of the Index that you want to remove.

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When should you use Index in SQL?

Accessing data from the large Database is a time-consuming and slow process. So, here comes the role of indexes. Indexes are used to easily retrieve the data from the Database.

So, if you want to frequently access some particular data from the Database. Then you can create an index of the Data to get easy access to that Data.

Therefore, it helps in increasing the overall performance of a Database.

When should you not use Index in SQL?

You should avoid using Indexes in the following cases:

1. You should not use Indexes for the small tables in a Database.

2. Since, the Index is the copy of the original Data. So, if the original Data gets modified, then the index will also get updated separately.

So, you should not use Index for the table in which you are going to perform manipulation or the changes in data frequently.

3. You should not use Index for the table having a large number of NULL values.

Conclusion

In this article, we have covered all the important topics that you should know to get a better understanding of Indexes in SQL. The topics which we have covered are:
  • Definition of SQL Index
  • Some basic commands to create Index in SQL
  • Command to remove the Index in SQL
  • Where should you use and where to avoid using Index in SQL.

So, hope you like the article on Indexes in SQL. We also have a blog post on SQL vs NoSQL, which you must take a look at.

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