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Asked: 2024-09-27T00:22:40+05:30 In:

how can we delete duplicate rows in sql

Hi there! I hope you can help me with a frustrating issue I’m currently facing in my SQL database. I’ve been working with a dataset that seems to have a lot of duplicate rows, and it’s really cluttering my results. I want to clean this up to ensure that my queries return only unique records.

However, I’m unsure of the best approach to effectively delete these duplicates without losing important data. I know that there are various ways to identify and remove duplicates, but I’m a bit overwhelmed by the options. For instance, should I use a temporary table? Or perhaps I can utilize the `ROW_NUMBER()` function to help distinguish between the original and duplicate entries?

I also worry about how this might affect data integrity and relationships with other tables. Is there a safe method to perform this operation, especially if I need to keep certain columns but remove complete duplicates across the entire row? Any guidance or examples on how to write the SQL query for this would be immensely appreciated! Thank you!

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    2 Answers

    1. How to Delete Duplicate Rows in SQL

      Okay, so you have a database and, uh-oh, you’ve got duplicate rows. Don’t worry! Here’s a simple way to get rid of them.

      Step 1: Find Duplicates

      First, you wanna find out which rows are duplicates. You can do this with a query. It looks something like this:

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

      This will show you the duplicates based on column1 and column2. Change these to whatever columns you need!

      Step 2: Delete Duplicates

      Now, to delete the duplicates, you can use a common table expression (CTE) if your SQL version supports it. Here’s how you do it:

      WITH CTE AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY (SELECT NULL)) AS rn
    FROM your_table
)
DELETE FROM CTE WHERE rn > 1;

      This basically keeps the first occurrence and deletes the rest. Make sure to replace column1 and column2 with your actual column names!

      Note!

      Before you run the delete command, it’s a good idea to back up your data or test on a small portion. Things can go south quickly!

      And that’s it! You should be good to go with less clutter in your database!


    2. To delete duplicate rows in SQL while preserving one instance of each duplicate, a common technique is to utilize a Common Table Expression (CTE) or a subquery combined with a DELETE statement. For instance, if you have a table named `my_table`, you can first identify the duplicates by using the ROW_NUMBER() window function. This function assigns a unique sequence number to each row within a partition of your dataset, allowing you to distinguish the duplicates. The query would look like this:

      “`sql
      WITH CTE AS (
      SELECT *, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY (SELECT NULL)) AS RowNum
      FROM my_table
      )
      DELETE FROM CTE WHERE RowNum > 1;
      “`
      In this example, `column1` and `column2` represent the columns you want to check for duplicates. The CTE filters out the duplicates by defining the conditions in the PARTITION BY clause, and the DELETE statement subsequently removes any rows where the row number exceeds 1.

      Another approach involves using a temporary table or a self-join. You can create a new table to store the distinct records and then delete all records from the original table before reinserting the unique entries. Here’s a generalized version of this approach:

      “`sql
      CREATE TABLE temp_table AS
      SELECT DISTINCT * FROM my_table;

      DELETE FROM my_table;

      INSERT INTO my_table SELECT * FROM temp_table;

      DROP TABLE temp_table;
      “`
      This method is particularly useful when you’re dealing with large datasets, as it directly redresses the data integrity without the overhead of window functions or multiple passes over the data.

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