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Asked: 2024-09-26T20:12:07+05:30 In:

how to delete the duplicate rows in sql

I’ve been working on cleaning up a database for my project, but I’ve run into a frustrating issue with duplicate rows. It’s a table that stores user information, and I’ve noticed some entries appear multiple times, which is causing discrepancies when I run reports and queries. I need to ensure that each user is represented only once, but I’m not entirely sure how to go about deleting those duplicates without accidentally losing important data.

I’ve done some research and learned there are various methods to tackle this problem, but I’m worried about the complexity of the SQL commands. I’m particularly concerned about maintaining the integrity of the remaining data. Should I create a temporary table to hold the unique records before attempting to delete the duplicates? Or is there a more straightforward method to achieve this?

Moreover, I’d like to know if there’s a way to specify which duplicate row to keep based on certain criteria, like the latest signup date or the highest account balance. Any guidance on the best practices for identifying and removing these duplicate entries in SQL would be greatly appreciated! Thank you!

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    2 Answers


    1. To delete duplicate rows in SQL effectively, you can utilize a Common Table Expression (CTE) along with the ROW_NUMBER() window function. This function allows you to assign a unique sequential integer to rows within a partition of a result set, ordering them based on the desired criteria. Here’s a general example using a sample table called `my_table`:

      “`sql
      WITH CTE AS (
      SELECT *,
      ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY (SELECT NULL)) as row_num
      FROM my_table
      )
      DELETE FROM CTE WHERE row_num > 1;
      “`

      In this query, the `PARTITION BY` clause identifies duplicate rows based on specified columns (`column1`, `column2`), while `ORDER BY (SELECT NULL)` ensures that no specific order is imposed on duplicates. The row numbers are assigned, and any rows with a `row_num` greater than 1 indicate duplicates that can be safely deleted. Alternatively, if you prefer a less complex method and your SQL platform supports it, you can also use the DELETE statement combined with a subquery. However, the CTE approach is generally more robust and adaptable to various situations, particularly in large datasets or when additional filtering is necessary.

    2. Deleting Duplicate Rows in SQL

      So, like, if you have a table and you notice there are some rows that are like, totally the same, here’s a simple way to get rid of them. It sounds kinda confusing at first, but bear with me!

      First, you might want to check which rows are duplicates. You can do 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 what you choose as columns. Just replace column1 and column2 with the actual names of your columns.

      Now, to actually delete those pesky duplicates, it’s often suggested to use a temporary table. It sounds fancy, but it’s not too hard!

      CREATE TABLE temp_table AS 
SELECT DISTINCT *
FROM your_table;

      This will create a temp_table that only has unique rows from your original table. Neat, right?

      Then you can just drop the old table and rename the new one:

      DROP TABLE your_table; 
ALTER TABLE temp_table RENAME TO your_table;

      And boom! Your table should now only have unique rows. Just remember to be careful with this stuff – you don’t wanna accidentally delete important data!

      Oh, and always, always back up your data before you start messing around. You know, just in case!

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