How to Identify and Retrieve Duplicate Rows in SQL by Utilizing GROUP BY and HAVING

Описание: Learn how to effectively find and extract duplicate rows within SQL tables, even when one column contains multiple values.
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This video is based on the question https://stackoverflow.com/q/67919934/ asked by the user 'Ron Wynn' ( https://stackoverflow.com/u/16186059/ ) and on the answer https://stackoverflow.com/a/67919987/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Identify and Retrieve Duplicate Rows in SQL by Utilizing GROUP BY and HAVING

Selecting duplicate rows in SQL can be a common challenge, particularly when you have columns with multiple values that could indicate effective duplicates. If you're seeking to solve this challenge, you're in the right place! In this guide, we will break down a straightforward solution to retrieve duplicates from your SQL table using the GROUP BY and HAVING clauses and row_number() function.

The Problem: Finding Duplicates

Imagine a scenario where you have a table containing customer order details. Your table looks something like this:

IDCust# Order# ItemCodeDataPoint1DataPoint21001123Ixxxyyyxxx1234562001123Insertxxxyyyxxx1234563001123Deleteasdf99994001123Dasdf9999In this example, the first two rows are considered duplicates because they can be identified by the Cust# , Order# , and closely related ItemCode. Similarly, rows three and four are also duplicates based on the same criteria. The challenge is to write a SQL statement that will allow you to retrieve specific rows based on whether they are duplicates while giving preference to rows with the higher ID value.

The Solution: SQL Select Statement

To effectively find and retrieve these rows, we can use a combination of SELECT, row_number(), and PARTITION BY functions. Here's how to construct your SQL query:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the SQL Query

ROW_NUMBER(): This function assigns a sequential integer to rows within a partition of a result set, starting at 1 for the first row in each partition.

PARTITION BY: This is used to divide the result set into partitions to which the ROW_NUMBER() function is applied. In our query, we're partitioning the data by Cust# , Order# , and the first character of ItemCode.

ORDER BY ID ASC: This specifies that we want to order the rows within each partition by the ID in ascending order. This ensures that the duplicates with the highest ID will come last in the sequence.

WHERE seqnum 1: Finally, we filter the results to select only those rows that have duplicates, effectively giving us the rows that are indicated as duplicates.

Important Notes

Keep in mind that not all SQL databases support the LEFT() function, but they generally have equivalent functions that can achieve the same purpose.

This approach assumes that the first character of the ItemCode is adequate to define identical rows for the purpose of identifying duplicates.

Conclusion

In conclusion, by using the method outlined above, you'll possess the tools necessary to identify and handle duplicate data in SQL effectively. Whether you're cleaning up a data table or needing to analyze customer orders more accurately, understanding how to leverage GROUP BY, HAVING, and the ROW_NUMBER() function will streamline your SQL queries.

Feel free to try out the code and adjust it to fit your specific database schema and data requirements. Happy querying!