SQL Unique Index

A SQL Unique Index is a constraint that ensures no two rows in the indexed columns of a table have the same values, thereby preventing duplicate entries. Unique indexes are crucial for maintaining data integrity, particularly when you need specific columns or combinations of columns to hold only unique values.

In this article, we will explore the concept of SQL Unique Indexes, their syntax, use cases, and practical examples with detailed explanations. By the end, you’ll have a clear understanding of how and when to use unique indexes to improve your database design and ensure data consistency.

What is a SQL Unique Index?

A SQL Unique Index is a database constraint that ensures the values in the indexed column(s) are unique across all rows in a table. It can be applied to one or more columns, ensuring data integrity by preventing duplicate values. Unique indexes can be created for both single columns and combinations of multiple columns. Additionally, they automatically exclude NULL values if multiple rows contain NULL in the indexed columns, further maintaining data consistency.

Key Characteristics of SQL Unique Index

• If a unique index is created on a single column, all rows in that column must be unique.
• If the unique index is created on multiple columns, the combination of values in these columns must be unique.
• A unique index cannot be created on a column if duplicate values already exist.
• Unique indexes improve query performance by optimizing data retrieval.

Syntax:

CREATE UNIQUE INDEX index_name
ON table_name (column1, column2, ..., columnN);

Key Terms:

• index_name: The name of the unique index.
• table_name: The name of the table where the index will be created.
• column1, column2, ..., columnN: The columns on which the unique index is being applied.

Examples of SQL Unique Index

Let’s create a sample CUSTOMERS table containing customer details such as ID, NAME, AGE, ADDRESS, and SALARY. This table serves as the basis for demonstrating the creation and use of unique indexes. Each row represents a unique customer, and we will apply unique constraints to specific columns to illustrate their functionality.

Query:

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR(15) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS VARCHAR(25),
   SALARY DECIMAL(10, 4),
   PRIMARY KEY(ID)
);

INSERT INTO CUSTOMERS VALUES 
(1, 'Ramesh', 32, 'Ahmedabad', 2000),
(2, 'Khilan', 25, 'Delhi', 1500),
(3, 'Kaushik', 23, 'Kota', 2000),
(4, 'Chaitali', 26, 'Mumbai', 6500),
(5, 'Hardik', 27, 'Bhopal', 8500),
(6, 'Komal', 22, 'Hyderabad', 9000),
(7, 'Muffy', 24, 'Indore', 5500);

Example 1: Creating a Unique Index on a Single Column

This example creates a unique index on the NAME column to prevent duplicate names in the CUSTOMERS table. It ensures that every entry in the NAME column is distinct, enforcing data integrity.

Query:

CREATE UNIQUE INDEX UNIQUE_NAME ON CUSTOMERS(NAME);

Output:

Explanation:

The output confirms the successful creation of the unique index on the NAME column. No rows are affected since the index is applied to existing data, and any future attempts to insert duplicate values will result in an error.

Example 2: Creating a Unique Index on Multiple Columns

This example demonstrates how to enforce uniqueness across a combination of two columns by creating a unique index on NAME and AGE.

Query:

CREATE UNIQUE INDEX MUL_UNIQUE_INDEX ON CUSTOMERS(NAME, AGE);

Output:

Query OK, 0 rows affected

Explanation:

The unique index MUL_UNIQUE_INDEX ensures that the combination of NAME and AGE values is unique for every row in the CUSTOMERS table. If you attempt to insert or update a row with the same combination of NAME and AGE values as an existing row, the database engine will throw an error, ensuring no duplicate combinations are allowed.

Example 3: Attempting to Create a Unique Index on a Column with Duplicate Values

This example illustrates the behavior of the database when trying to create a unique index on a column that already contains duplicate values.

Query:

CREATE UNIQUE INDEX UNIQUE_SALARY ON CUSTOMERS(SALARY);

Output:

| ERROR 1062 (23000): Duplicate entry '2000.00' for key 'customers.UNIQUE_SALARY' |

Explanation:

The database engine prevents creating a unique index on the SALARY column because it contains duplicate entries (2000.00 appears twice). To resolve this issue, we would need to remove or update duplicate values.

Example 4: Verifying Indexes in a Table

This example demonstrates how to verify the unique indexes created on the CUSTOMERS table using the SHOW INDEX command.

Query:

SHOW INDEX FROM CUSTOMERS;

Output:

Explanation:

The SHOW INDEX command lists all indexes created on the table. It displays the index names, the associated columns, and their properties. This information helps verify the existence and structure of indexes applied to the table, ensuring their proper implementation.

Example 5: Handling Duplicate Entries in Indexed Columns

This example demonstrates what happens when an UPDATE operation attempts to assign a duplicate value to a column with a unique index.

Query:

UPDATE CUSTOMERS SET ADDRESS = 'Mumbai' WHERE ADDRESS = 'Delhi';

Output:

 | ERROR 1062 (23000): Duplicate entry 'Mumbai' for key 'customers.ADD_UNIQUE_INDEX' |

Explanation:

The UPDATE query tries to change the value of ADDRESS from 'Delhi' to 'Mumbai', but the column ADDRESS has a unique index (ADD_UNIQUE_INDEX). Since 'Mumbai' is already present in another row, the database engine prevents this operation to maintain the uniqueness constraint on the ADDRESS column.

Benefits of SQL Unique Index

• Data Integrity: A unique index ensures that no duplicate values exist in the indexed column(s), maintaining the accuracy and reliability of your data.

• Improved Query Performance: Unique indexes optimize data retrieval for queries involving unique values, making searches faster and more efficient.
  
• Flexibility: Unique indexes can be applied to single columns or a combination of multiple columns, offering versatility in enforcing constraints across different data structures.
  
• Error Prevention: Unique indexes help prevent unintentional duplicates during INSERT or UPDATE operations, safeguarding your database from inconsistent or invalid data

Conclusion

The SQL Unique Index is a powerful tool for maintaining data integrity and optimizing query performance. By enforcing uniqueness in one or more columns, it ensures that our database remains consistent and reliable. Use the CREATE UNIQUE INDEX statement strategically to prevent duplicate entries and improve data retrieval efficiency. With examples and detailed explanations, this guide provides everything we need to understand and implement unique indexes effectively.