Home/ Questions/Q 14677
Next
In Process

askthedev.com Latest Questions

Asked: 2024-09-27T03:24:31+05:30 In:

how to delete duplicate data in sql

I’m currently working with a database where I’ve noticed that there are multiple duplicate entries in one of my tables, and it’s really causing issues with my data integrity. I have a customer table that includes columns like customer_id, name, email, and phone number. Unfortunately, due to some errors during data entry and imports, I have several rows with identical details. It’s becoming increasingly difficult to analyze this data accurately or even to generate meaningful reports.

I’ve tried a few basic queries to identify the duplicates, but I’m unsure how to go about actually deleting them without losing any valuable information. For example, I want to make sure that I keep one instance of each duplicate entry while removing the rest. Should I use a DELETE statement with a JOIN or perhaps a subquery? I’ve also heard of using the ROW_NUMBER() function, but I’m not quite sure how to implement it correctly. Can anyone guide me through the best practices for deleting duplicate rows in SQL while ensuring that the remaining data is clean and accurate? Thank you!

Share

    Leave an answer

    You must login to add an answer.

    Need An Account,

    2 Answers

    2. First, figure out what table has those duplicates. Let’s say it’s called my_table.
    3. Next, you can try using a simple SELECT statement to see what the duplicates look like. Something like:
      4. SELECT column1, column2, COUNT(*) 
FROM my_table 
GROUP BY column1, column2 
HAVING COUNT(*) > 1;
    4. Now you’ll know which ones are duplicates. But how do we delete them? You’ll need a way to keep one and get rid of the others. So, you can do something like:
      5. DELETE FROM my_table 
WHERE id NOT IN (
    SELECT MIN(id) 
    FROM my_table 
    GROUP BY column1, column2
);
    5. This means it’ll keep the row with the smallest id and delete the rest. Make sure id is something that uniquely identifies each row!
    6. But be super careful! Always back up your data first before running delete commands. I totally don’t want you to accidentally lose important stuff.
    7. And, umm, if you’re unsure, maybe just work in a test database until you get the hang of it!

    So yeah, that’s kinda the gist of it! Good luck!


  • To efficiently delete duplicate data in SQL, one of the common approaches involves using the Common Table Expressions (CTE) with the ROW_NUMBER() window function. This allows you to assign a unique sequential integer to rows within a partition of a result set, thereby enabling the identification of duplicate records. For instance, you can execute a query that ranks the rows based on specific criteria (like an ID or timestamp) and then filter to retain only the first occurrence of each duplicate. The SQL command would look something like this:

    “`sql
    WITH CTE AS (
    SELECT *,
    ROW_NUMBER() OVER (PARTITION BY column_name ORDER BY id_column) AS row_num
    FROM your_table
    )
    DELETE FROM CTE WHERE row_num > 1;
    “`

    In this example, replace `column_name` with the name of the column you want to check for duplicates, and `id_column` would typically be a unique identifier for your records. Another method is utilizing the DELETE statement in conjunction with a subquery that targets duplicates, often involving a GROUP BY clause combined with a HAVING statement. This method also ensures that a duplication check is performed without utilizing a CTE. Both methods are effective, but the choice ultimately depends on your specific database system’s capabilities and performance characteristics.

    • Related Questions