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Asked: 2024-09-27T04:25:27+05:30 In:

how to delete a duplicate rows in sql

I’ve been working on a project where I need to manage a database containing a large amount of data, and I’ve encountered a frustrating issue with duplicate rows in one of my tables. It’s not just a few duplicates; there are hundreds of them, and it’s causing problems for my queries and data analysis. I’ve looked into some options, but I’m unsure about the best approach to effectively delete these duplicates without affecting the data integrity.

I understand that each duplicate row has the same values for certain columns, but there might be unique identifiers or timestamps in other columns. How can I identify the duplicates accurately? Should I use a temporary table to store unique records first, or is there a more direct way to delete duplicates? Additionally, I’m concerned about how this might impact related tables if there are foreign key relationships. Can someone provide a step-by-step method to tackle this issue? Any specific SQL queries or examples would be greatly appreciated, as I want to ensure I do this correctly and efficiently. Thank you!

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    2 Answers

    1. Deleting Duplicate Rows in SQL

      Ok, so you have this table and you see some duplicate rows. Like, it’s super annoying, right? Here’s a simple way to sort this out, even if you’re just getting started with SQL.

      Imagine you have a table called my_table with some duplicates. First, you need to figure out what makes a row duplicate. Is it all the columns, or just some? Let’s say it’s the whole row.

      One way to delete duplicates is to use a DELETE command with a little help from a ROW_NUMBER(). But, like, we’ll break it down:

              
            DELETE FROM my_table
            WHERE id NOT IN (
                SELECT id FROM (
                    SELECT id, 
                           ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY id) as rn
                    FROM my_table
                ) AS temp
                WHERE rn = 1
            );

      Here’s what’s happening:

      • ROW_NUMBER() gives each row a number based on duplicates (like 1, 2, 3 for the same thing).
      • PARTITION BY is like saying, “Hey, check these columns for duplicates.”
      • We only want to keep the first one (the one with rn = 1).
      • Then we’re deleting all the others using their IDs.

      Just make sure to back up your stuff first, because deleting kinda feels permanent, ya know? Hope this helps a bit!


    2. To effectively delete duplicate rows in SQL, one of the most common approaches is to utilize a Common Table Expression (CTE) along with the `ROW_NUMBER()` window function. This method allows you to assign a unique sequential integer to rows within a partition of a result set, effectively distinguishing between original and duplicate entries. Here’s an example using a table named `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, replace `column1` and `column2` with the actual column names that identify duplicates. The `PARTITION BY` clause groups the rows based on those columns while the `ORDER BY (SELECT NULL)` helps in defining a nonspecific order for assigning row numbers, thus keeping the first occurrence of each duplicate and marking subsequent duplicates for deletion. As always, it is prudent to test your deletion strategy on a sample dataset first or use a transaction to ensure you can roll back if errors occur.

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