Скачать или смотреть How to Use SELECT DISTINCT on Multiple Columns in SQL

Learn how to effectively retrieve unique combinations of multiple columns in SQL. This guide walks you through using `SELECT DISTINCT` and offers a practical example to manage duplicates in your sales data.
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This video is based on the question https://stackoverflow.com/q/54418/ asked by the user 'sheats' ( https://stackoverflow.com/u/4915/ ) and on the answer https://stackoverflow.com/a/54430/ provided by the user 'Joel Coehoorn' ( https://stackoverflow.com/u/3043/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Challenge: Selecting Distinct Rows on Multiple Columns

When working with databases, it is common to encounter situations where you need to retrieve unique records based on the values of multiple columns. This can be particularly essential in cases such as analyzing sales data, where you may want to ensure that each entry for a specific day and price combination is unique.

For instance, consider a sales database where you want to update the status of sales records that are unique regarding the sales price and the date of sale. The challenge lies in selecting the rows effectively without returning duplicates based on your criteria.

The Problem Statement

In your case, the requirement is to update the status of sales records to "ACTIVE" if each combination of saledate (the date of sale) and saleprice (the price of sale) appears only once in the dataset. This means you want to avoid any cases where the same price appears on the same day more than once.

Your first instinct might lead you to use a subquery to select distinct values, but this can often become complex. Let’s explore a suitable solution.

Crafting the Solution

To achieve the desired update, we can utilize a combination of GROUP BY, INNER JOIN, and HAVING clauses. Let’s break this down step by step to understand how to structure your SQL query effectively.

Step 1: Understanding SELECT DISTINCT and GROUP BY

While you might think of using SELECT DISTINCT for this purpose, a more powerful approach is using GROUP BY, which allows for additional aggregate functions to be used easily.

Here’s a basic illustration of how SELECT DISTINCT is commonly applied, which can also be expressed using GROUP BY:

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

This is roughly equivalent to:

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

However, in your case, we can put together a more practical query.

Step 2: The Final Query Explained

Here’s a refined query that accomplishes your goal:

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

Breakdown of the Query

Subquery with GROUP BY:

The inner subquery groups sales records by saleprice and saledate.

It uses HAVING COUNT(*) = 1 to filter only those combinations that occur once—ensuring their uniqueness.

Inner Join:

The main query joins the sales table with the results of the subquery on the conditions that match both saleprice and saledate.

Update Statement:

Finally, it updates the status of those unique sale records to 'ACTIVE'.

Conclusion

This method provides a systematic way to address the need for selecting distinct combinations of multiple columns in SQL. By utilizing GROUP BY and HAVING, you can efficiently manage duplicates in your datasets, tailoring your approach based on exact business needs.

In summary, understanding how to structure your SQL queries to ensure data integrity not only makes your code cleaner but also enhances your database management capabilities.

If you ever find yourself needing to handle similar scenarios, you can refer back to this structured approach!