December 4, 2023

 Eliminating Duplicate Entries in SQL Best Practices and Techniques

4 min read
how to exclude duplicates in sql


In the realm of database management, dealing with duplicate entries is a common challenge. Duplicates can cause inefficiencies, hinder data accuracy, and complicate data analysis. However, SQL provides several powerful techniques and strategies to exclude duplicates effectively. This article aims to explore these methods, shedding light on the best practices for eliminating duplicate entries in SQL.

Understanding Duplicate Data

Duplicate data refers to identical or nearly identical records that exist in a database. These duplicates can occur due to various reasons, such as data entry errors, system glitches, or faulty integration processes. Before proceeding to eliminate duplicates, it is crucial to comprehend the nature of the duplicate data in question.

Primary Keys and Unique Constraints

To prevent duplicate entries, one of the fundamental practices is to define primary keys and unique constraints in database tables. A primary key uniquely identifies each record in a table, ensuring its uniqueness. Similarly, unique constraints define columns that must contain unique values. Utilising primary keys and unique constraints helps enforce data integrity at the database level, significantly reducing the chances of duplicates.

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The DISTINCT keyword is a straightforward method to retrieve distinct values from a column in SQL queries. By incorporating the DISTINCT keyword, one can retrieve only unique records, excluding duplicates. For example, to select unique employee names from an “employees” table, the query would be

SELECT DISTINCT employee_name FROM employees


The GROUP BY clause is a powerful tool for grouping rows based on common attributes and performing aggregate functions on those groups. When used with aggregate functions such as COUNT, SUM, or AVG, the GROUP BY clause can effectively eliminate duplicates and provide summary information. For instance, to count the number of orders placed by each customer while excluding duplicates, the query would be


SELECT customer_id, COUNT(order_id) 

FROM orders 

GROUP BY customer_id;



Subqueries are queries embedded within another query, allowing for complex and efficient data retrieval. They can be employed to exclude duplicates by filtering out records that match specific criteria. Subqueries can be utilised in various SQL statements such as SELECT, INSERT, UPDATE, or DELETE. For instance, to select customers who have not placed any orders, the following subquery can be used

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SELECT customer_id, customer_name 

FROM customers 

WHERE customer_id NOT IN (SELECT DISTINCT customer_id FROM orders);


JOIN Operations

JOIN operations are used to combine rows from two or more tables based on related columns. By joining tables appropriately, duplicates can be excluded or consolidated. INNER JOIN, LEFT JOIN, and RIGHT JOIN are common types of JOIN operations. For instance, to retrieve a list of products along with their corresponding categories while excluding duplicate products, the following query can be used


SELECT p.product_name, c.category_name 

FROM products p

JOIN categories c ON p.category_id = c.category_id;


Temporary Tables and Views

Temporary tables and views can be utilised to store intermediate results during the duplicate exclusion process. They allow for complex queries to be broken down into smaller, manageable steps. By using temporary tables or views, it becomes easier to perform operations such as filtering, joining, or grouping, ultimately leading to the exclusion of duplicates.

Frequently Asked Questions

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How do you exclude duplicates in SQL query?

One of the easiest ways to remove duplicate data in SQL is by using the DISTINCT keyword. You can use the DISTINCT keyword in a SELECT statement to retrieve only unique values from a particular column.

How do I remove all duplicates but keep only one in SQL?

First, the CTE uses the ROW_NUMBER() function to find the duplicate rows specified by values in the first_name , last_name , and email columns. Then, the DELETE statement deletes all the duplicate rows but keeps only one occurrence of each duplicate group.


Duplicate entries in databases can impede data quality and hinder efficient data analysis. However, by implementing the techniques outlined in this article, such as primary keys, unique constraints, DISTINCT keyword, GROUP BY clause, subqueries, JOIN operations, and temporary tables/views, one can effectively exclude duplicates in SQL. It is essential to choose the most appropriate method based on the specific requirements and complexity of the data. By employing these best practices, data professionals can ensure data accuracy, enhance query performance, and streamline database operations.

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