How to Delete Duplicate Records in SQL?

In this blog, we will see How to Delete Duplicate Records in SQL? in steps to find the exact duplicates and remove them using SQL query.

Duplicate records can cause a variety of problems, such as incorrect calculations and reporting, incorrect data analysis, and reduced performance.

What are the Duplicates Records in SQL?

Duplicate records in SQL refer to rows in a table that have the same values in one or more columns. In other words, there are two or more rows in the table with the same data.

Duplicate records can be created due to various reasons, such as human error during data entry, bugs in applications, or the design of the database.

It’s important to remove duplicates to ensure data integrity and accuracy.

To identify and remove duplicates in SQL, you can use various techniques such as using the DISTINCT keyword in a SELECT statement, using the GROUP BY clause, or using temporary tables.

How to check Duplicate Records in a table in SQL?

To check for duplicate records in a table in SQL, you can use the following methods:

1. Use the DISTINCT keyword:

You can use the DISTINCT keyword in a SELECT statement to retrieve only the unique rows from a table. For example:

SELECT DISTINCT column1, column2, column3
FROM table_name;

2. Use the GROUP BY clause:

You can use the GROUP BY clause in a SELECT statement to group rows that have the same values in one or more columns. For example:

SELECT column1, column2, column3, COUNT(*)
FROM table_name
GROUP BY column1, column2, column3
HAVING COUNT(*) > 1;

This query will return the rows that have duplicates, along with the number of duplicates for each group of duplicates.

3. Use a self-join:

You can use a self-join to compare each row with every other row in the same table. For example:

SELECT t1.column1, t1.column2, t1.column3
FROM table_name t1
JOIN table_name t2
ON t1.column1 = t2.column1 AND t1.column2 = t2.column2 AND t1.column3 = t2.column3
WHERE t1.id < t2.id;

This query will return all the rows that have duplicates, based on the values in the specified columns.

Note that the exact syntax may vary depending on the database management system you are using.

Steps to Remove Duplicate records in SQL

To delete duplicate records in SQL, you can use the following steps:

1. Identify the duplicate records: You can do this by using a SELECT statement with the DISTINCT keyword or by using the GROUP BY clause.
2. Create a temporary table to store the unique records: Once you have identified the duplicate records, you can create a temporary table to store the unique records.
3. Populate the temporary table with the unique records: You can do this by inserting the unique records from the original table into the temporary table.
4. Drop the original table: After you have populated the temporary table with the unique records, you can drop the original table.
5. Rename the temporary table: Finally, you can rename the temporary table to the name of the original table to replace the original table with the unique records.

Here is an example that demonstrates these steps using the SQL Server database management system:

WITH CTE AS
(
    SELECT MIN(id) AS min_id, column1, column2, column3
    FROM original_table
    GROUP BY column1, column2, column3
)
SELECT * INTO temp_table
FROM original_table
WHERE id IN (SELECT min_id FROM CTE);

DROP TABLE original_table;

EXEC sp_rename 'temp_table', 'original_table';

Why does it require removing Duplicates from the table?

There are several reasons why it’s important to remove duplicates from a table in SQL:

1. Data Integrity: Duplicates can cause data to become inconsistent and unreliable, leading to incorrect results in calculations, reporting, and data analysis.
2. Storage Space: Duplicates take up extra storage space, which can lead to decreased performance and increased cost over time.
3. Performance: When duplicates are present, SQL queries can take longer to execute and return results, as the database needs to process and filter through the extra data.
4. Data Quality: Duplicates can cause confusion and misinterpretation, leading to incorrect decisions based on the data.
5. Compliance: In some industries, it may be a requirement to maintain unique records to comply with regulations and standards.

Removing duplicates is an important step in data cleaning and preparation, and it helps ensure that the data in a table is accurate, reliable, and of high quality.

Conclusion

In conclusion, removing duplicates from a table in SQL is an important step in maintaining data quality and integrity.

Duplicate records can cause a variety of problems such as incorrect calculations, decreased performance, and data inconsistency.

By using techniques such as the DISTINCT keyword, the GROUP BY clause, or temporary tables, you can identify and remove duplicates in SQL, ensuring that your data is accurate and reliable.