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Asked: 2024-09-27T01:11:40+05:30 In:

how to delete duplicate rows in sql

I’ve been working on my SQL database, and I’ve run into a frustrating issue with duplicate rows. It seems like every time I insert new data or update existing records, duplicate entries keep popping up. For example, I have a table that tracks customer information, and I noticed that several customers have been listed multiple times for the same details. This is obviously not ideal, as it could lead to confusion and inaccuracies in reporting.

I’ve tried doing simple SELECT queries to identify duplicates based on unique fields like email addresses and IDs, but I’m struggling with how to actually delete those duplicates from the table. Should I be using a DELETE statement, or is there a more efficient method to clean up these rows? I’ve heard people mention using common table expressions (CTEs) or temporary tables, but I’m not quite sure how to implement them to achieve my goal.

Can someone give me a clear explanation of the best approach to effectively remove those duplicate rows while keeping one original entry? Any sample queries or methods you recommend would be greatly appreciated!

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    2 Answers

    1. Deleting Duplicate Rows in SQL

      So, like, if you have a table with some rows that are the same and you wanna get rid of them, here’s a simple way to do it! 💻✨

      First, you gotta figure out which table has these duplicates. Let’s say it’s called my_table.

      Now, you can use something called a Common Table Expression (CTE). Just copy and paste this code:

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

      Okay, here’s what’s happening:

      • We’re making a CTE called CTE that looks at my_table.
      • Then, we select everything and give each row a number (rn) based on what columns you wanna check for duplicates. Replace column1 and column2 with the actual names of your columns.
      • Finally, we delete rows where the number is greater than 1, so it keeps just one of each duplicate.

      Remember to back up your data before running delete commands! You don’t wanna mess stuff up. 😱

      If you’re just starting out, maybe try this on a test table first to see how it works!


    2. To delete duplicate rows in SQL, a common approach is to use a Common Table Expression (CTE) or a subquery in conjunction with the DELETE statement. First, identify the unique column(s) that constitute the duplicate criteria. You can use a CTE to isolate the duplicate records while assigning a row number to each duplicate group based on specific criteria. For example, in databases like PostgreSQL or SQL Server, you can use the ROW_NUMBER() function to enumerate the duplicates. Once you’ve created this CTE, you can proceed to delete rows where the row number is greater than one, effectively retaining just one instance of each duplicate.

      Here’s a generic SQL template you can follow:
      “`sql
      WITH CTE AS (
      SELECT *,
      ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY (SELECT NULL)) as row_num
      FROM your_table
      )
      DELETE FROM CTE WHERE row_num > 1;
      “`
      In this example, replace `your_table` with the name of your table, and `column1, column2` with the columns that define your duplicates. Keep in mind that the `ORDER BY (SELECT NULL)` clause is used to remove any biases in row number assignment; however, it can be tailored to suit your sorting needs if there’s a specific criterion for which duplicate to keep. In other databases like MySQL, you may need to adopt a slightly different approach using a temporary table or grouping followed by deletion to achieve similar results.

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