How To Find Duplicates In SQL?

By: ninad magdum | July 5, 2023 | 9 mins read

According to SQL, duplicate rows in a table have the same entries in one or more columns. It is a common task to identify and handle duplicates in databases, as they can lead to inconsistent information and affect the accuracy of analytical results.

To avoid or manage duplicate records, it is critical to enforce data integrity rules such as defining unique constraints or primary keys, implementing proper data validation, and performing deduplication checks. Perform this task before inserting new data.

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How To Find Duplicates In SQL

Finding duplicate values in SQL is critical for preserving data integrity, cleansing data, optimizing performance, and aligning with specific business logic needs. It contributes to more accurate and consistent data, improved query performance, and informed decision-making.

By increasing the amount of data to analyze and reducing the effectiveness of indexes, duplicates can have an influence on query performance. You can optimize query execution and database performance by detecting and dealing with duplicates.

In some circumstances, duplicates are not faults but have important business implications. For example, you may need to identify duplicate customer records in order to merge them or track duplicate transactions in order to reconcile them.

Functions to find duplicates in SQL

There are many resources and capabilities that can help when it comes to detecting duplication in SQL. Here are some keys that are frequently used to find duplicates:

DISTINCT Keyword: The DISTINCT keyword retrieves unique data from a specified column. It can help you find distinct values and probable duplicates fast.
COUNT() Function: The COUNT() function returns the number of occurrences of a particular column or expression. It can be used to determine the frequency of values in a column, which can help in the identification of duplicate occurrences.
GROUP BY Clause: The GROUP BY clause allows you to group rows based on one or more columns. It is commonly used along with aggregate functions like COUNT, SUM, or AVG to perform calculations on grouped data. You can discover duplicates based on specified columns and analyze how they occur by grouping data.
HAVING Clause: The HAVING clause is used in conjunction with the GROUP BY clause to filter groups depending on a condition. It allows you to define conditions on the aggregated values, such as counting the number of occurrences, to return just the groups that fulfill the stated criteria (e.g., duplicates).
Self-Joins: Self-joins involve combining two tables based on a common column. Self-joins can let you compare rows within a database and find duplicates based on specific requirements or matching criteria.
Subqueries: Subqueries allow you to make nested queries within a main query. They enable you to do intermediate calculations or access specific data sets. Subqueries can aid in the detection of duplication by comparing values from distinct sets or circumstances.
Indexing: Proper indexing of important data can increase the efficiency of duplicate identification queries greatly. Indexes enable faster value retrieval and comparison, especially when dealing with huge datasets.
Data Cleansing Tools: Various database management tools and software have functions to identify and handle duplicates. These solutions frequently use powerful algorithms, fuzzy matching approaches, and customizable settings to help you detect and resolve duplicates faster.

Why do we use SQL to find duplicate values?

We use SQL to identify duplicate values because it offers a powerful and efficient method for querying and manipulating relational databases. Here are some of the reasons why SQL is frequently used to discover duplicates:

Standardized Query Language: SQL is a standardized language for working with relational databases. It defines a consistent syntax and set of commands for usage with various database management systems (DBMS). This makes it easier to create and run queries for identifying duplicates regardless of the database system used.
Data Manipulation Capabilities: SQL provides a plethora of strong capabilities and operations for data manipulation. It has operations like COUNT (), GROUP BY, and HAVING that are useful for detecting duplication. These functions enable you to execute aggregations, grouping, and filtering operations on your data, making it simple to identify and analyze duplicate values.
Efficiency and Performance: SQL has been optimized for database operations and can handle massive datasets efficiently. The underlying database engine is designed to process queries quickly, making it ideal for locating duplicates in tables containing millions or even billions of rows. SQL also enables indexing and optimization techniques, which improve query efficiency even further.
Flexibility and Customization: SQL gives flexibility and customization in terms of the queries you may construct to discover duplicates. You can provide the columns to be compared, create custom rules for detecting duplicates, and modify the query to meet your individual business needs. This adaptability allows you to customize the duplicate identification method to meet your individual requirements.
Integration with Database Systems: SQL integrates with a variety of database systems, including well-known ones like MySQL, Oracle, Microsoft SQL Server, and PostgreSQL.
These systems provide comprehensive SQL access and often incorporate additional capabilities and optimizations when working with large datasets. You can take advantage of the capabilities and performance optimization provided by these database systems by utilizing SQL.
Wide Adoption and Community Support: SQL is extensively embraced in the business, and there is a large community of developers and database specialists who are experienced with SQL and can provide support. This makes it easy to access information, lessons, and solutions to frequent SQL duplication difficulties.

How to write SQL query to find duplicate records

1. Create a Table

customers_id	first_name	last_name	age
1	John	Doe	22
2	David	Luna	25
3	Robert	Doe	25
4	John	Luna	29
5	David	Robinson	27
6	Betty	Doe	22

2. Find duplicates in SQL

i. Find duplicate names or values in a single column

Syntax

SELECT name, COUNT(name)
FROM table_name
GROUP BY name
HAVING COUNT(name) > 1;

Where,

SELECT: This keyword is used to define the columns in the result set to obtain.

name: It represents the precise column name from the table_name that contains the names to be checked for duplicates. You can replace the name with the actual column name in the table.

FROM table_name: The name of the table from which you want to retrieve data is indicated. Table_name should be replaced with the actual name of your table.

GROUP BY name: This clause groups the rows based on the name column. It makes separate groups for each unique name.

HAVING COUNT(name) > 1: The HAVING clause is used to filter the groups based on an aggregated value requirement. In this case, it specifies that only groups with a count (COUNT(name)) greater than one (i.e., duplicates) are accepted.

Example:

Query:

Output:

This is because “John” and “David” are the only two names that appear more than once in the table and satisfy the HAVING clause’s conditions.

ii. How many times are duplicate values present in multiple columns

Syntax:

SELECT column_name, COUNT(*) as duplicate_count
FROM table_name
GROUP BY column_name
HAVING COUNT(*) > 1;

Output:

John and David appear twice in the table. The duplicate_count column shows the number of times each name appeared.

iii. Find duplicate rows based on multiple columns

To find duplicate rows in multiple-column values, we can use the following query.

Syntax

SELECT column_name1, column_name2, COUNT(*)
FROM table_name
GROUP BY column_name1, column_name2
HAVING COUNT(*) > 1

Example

Query

SELECT last_name, age, COUNT(*) as duplicate_count
FROM customers
GROUP BY last_name, age
HAVING COUNT(*) > 1

Output

In this case, the query groups the rows depending on the combination of values in last_name and age.

Advantages of finding duplicates in SQL

Data Quality: Recognizing and dealing with duplicate records helps to assure data accuracy and reliability. Duplicates can cause discrepancies and inaccuracies in data analysis and reporting. You can improve data accuracy and reliability by identifying and fixing duplicates.
Data Deduplication: Locating duplicates allows you to delete or consolidate superfluous data. This procedure decreases the amount of storage space required and streamlines data maintenance. It also aids in the elimination of superfluous duplication, resulting in more streamlined and efficient data.
Query Performance: Duplicate records can impact query performance, especially when you are working with large datasets. By detecting and removing duplicates, you can enhance query execution times, system speed, and overall database efficiency.
Decision-Making: Accurate and trustworthy data is required for informed decision-making. You may ensure that decision-makers have access to clean, non-redundant data by spotting duplication. This increases the precision and efficacy of corporate insights and decision-making processes.
Data Cleansing: Duplicates may arise from various factors such as data entry errors, system glitches, or issues during data integration processes. Finding duplicates allows you to detect and correct data quality concerns. Cleaning out duplicates helps to keep data consistent and proper.
Data Integration: When you merge data from multiple sources, duplicates may appear. Identifying and resolving duplication is critical for data integration efforts. By recognizing matching records, you can merge and consolidate data from several sources, resulting in a single and consistent dataset.
Data Compliance: Duplicate records can have an impact on regulatory compliance. For example, in circumstances where duplicates can lead to double-counting or erroneous reporting, locating and correcting duplicates is required to meet compliance standards.
Improved User Experience: By reducing duplicates, you can improve the user experience in data-driven applications or systems. Working with cleaner and more trustworthy datasets benefits users, resulting in increased efficiency and productivity.

FAQs on how to find duplicates in SQL

Can duplicates affect database performance?

Yes, duplicates can adversely affect database performance by increasing storage requirements, reducing query performance, reducing index performance, and increasing maintenance overhead.

Can I use these techniques in any SQL database?

Yes, the procedures in this article are applicable to most SQL databases, including MySQL, PostgreSQL, Oracle, and SQL Server.

What is the most efficient way to find duplicates in SQL?

Answer: Use the GROUP BY and HAVING clauses to group rows by defined columns and filter out groups with a count larger than one.

How do you handle duplicates in SQL?

Use the update command with a subquery to update duplicate rows depending on specified columns, providing them with distinct values to make them unique.

How often should I check for duplicates in my databases?

The frequency with which you check for duplicates is determined by the nature of your data and the rate at which it is entered or updated. Duplicate checking should be incorporated into your regular data management procedure.

Final thoughts

In conclusion, by applying the techniques provided in this article, you can effectively find duplicates in the SQL database. Remember to regularly check for duplicates, set up data security regulations, and optimize your database performance for a smooth and efficient data management experience.

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