remove duplicate rows oracle

Guri Bee

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17-10-2024

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Removing Duplicate Rows in Oracle: A Comprehensive Guide

Duplicate rows can wreak havoc on your database integrity, leading to inaccurate analysis and inefficient queries. Thankfully, Oracle provides several methods to tackle this issue. This article will guide you through the most effective approaches, using insights from real-world discussions on GitHub.

Understanding the Problem

Duplicate rows arise when multiple entries in a table share the same values for all designated columns. This often happens due to data entry errors, merging datasets, or incomplete data cleansing.

Identifying Duplicate Rows

Before removing duplicates, it's crucial to identify them. Oracle provides powerful tools for this:

• The ROW_NUMBER() function: This function assigns a unique number to each row within a partition, allowing you to filter out duplicates based on specific criteria. For instance, to identify duplicate rows based on the customer_name and customer_id columns:

SELECT customer_name, customer_id, ROW_NUMBER() OVER (PARTITION BY customer_name, customer_id ORDER BY customer_id) as row_num
FROM customer_table;

• The DISTINCT keyword: This keyword filters out duplicate rows based on the specified columns. For example, to select unique customer names:

SELECT DISTINCT customer_name
FROM customer_table;

Methods for Removing Duplicate Rows

1. DELETE with Subquery: This approach leverages the DELETE statement with a subquery to target and remove duplicate rows based on specific criteria.

DELETE FROM customer_table
WHERE ROWID IN (
    SELECT ROWID
    FROM customer_table
    WHERE customer_name = 'John Doe' AND customer_id = 1234
    MINUS
    SELECT ROWID
    FROM customer_table
    WHERE customer_name = 'John Doe' AND customer_id = 1234
    GROUP BY customer_name, customer_id
    HAVING COUNT(*) > 1
);

Source: GitHub Discussion: Removing Duplicate Rows

2. Using ROW_NUMBER() for Targeted Deletion: This method combines ROW_NUMBER() with a DELETE statement to remove duplicate rows while preserving a single representative record.

DELETE FROM customer_table
WHERE ROWID IN (
    SELECT ROWID
    FROM (
        SELECT ROWID, ROW_NUMBER() OVER (PARTITION BY customer_name, customer_id ORDER BY customer_id) AS rn
        FROM customer_table
    )
    WHERE rn > 1
);

Source: GitHub Snippet: Removing Duplicates with ROW_NUMBER()

3. Deleting Duplicates Based on a Primary Key: If your table has a primary key, you can leverage it to identify and remove duplicates. This is a straightforward and efficient approach.

DELETE FROM customer_table
WHERE customer_id NOT IN (SELECT MIN(customer_id) FROM customer_table GROUP BY customer_name, customer_id);

Source: Oracle Documentation: Removing Duplicate Rows

Important Considerations

• Data Integrity: Before removing duplicates, double-check your data and ensure that your logic correctly identifies the desired rows for deletion.
• Backup: Always back up your data before executing any deletion operations.
• Constraints: If your table has constraints, ensure your deletion logic is compatible with them.

Conclusion

Removing duplicate rows in Oracle requires a careful approach. This guide provides a comprehensive overview of different techniques using insights from GitHub discussions and official documentation. By carefully selecting the appropriate method and double-checking your logic, you can effectively eliminate duplicates and maintain data integrity in your database.