How to Select DISTINCT Records Based on Multiple Columns in SQL Server

Learn how to effectively select `DISTINCT` records based on multiple columns in SQL Server to avoid duplicate entries while considering the data order.
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A Guide to Selecting DISTINCT Records Based on Multiple Columns in SQL Server

When working with databases, sometimes we encounter the need to retrieve records that meet certain criteria without returning duplicates. One common scenario is returning customer pairs from the same city. In this post, we'll explore how to achieve this in SQL Server effectively.

The Problem

You're given a table named Customer with the structure:

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And here's an example of the sample data you might encounter:

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The objective is to retrieve pairs of customers that live in the same city while ensuring that no duplicate pairs are returned. For instance, if we have both Foo Foo and Bar Bar as customers in New York, we only want to see one distinct row for them rather than having the swapped pair appear as well.

The Query You Tried

You started with a query that looked similar to this:

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However, this resulted in duplicated rows with swapped customer names:

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The Solution

To effectively solve this problem and achieve the desired result, we can modify your existing SQL statement. The key is to adjust our WHERE clause to ensure that we establish an order when comparing customer names, thus avoiding duplicates.

Step-by-Step Breakdown

Create a Table: First, let’s declare a sample table for our customers.

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Modify the Selection Query: We will adjust the selection logic by using a different comparison in our WHERE clause.

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Explanation of the Query

JOIN: We are joining the Customer table with itself based on the City column, which allows us to compare all customers within the same city.

WHERE Clause: The change here is critical. By using > instead of <>, we ensure that we only get one entry for each pair, with the condition ensuring that customer names are compared in a defined lexicographical order, effectively eliminating duplicates.

Expected Result

With this adjustment, the result from running the above code will look like this:

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Conclusion

Selecting distinct records based on multiple columns in SQL Server can seem daunting at first, especially when the records have potential duplicates in reversely paired formats. But with a slight modification to your SQL logic, you can easily achieve the desired output. By focusing on the ordering in your comparisons, you can effectively eliminate redundant entries and get cleaner results from your database queries.

With this guide, you should now feel more confident in constructing your SQL queries to handle similar problems efficiently. Happy querying!