Скачать или смотреть How to Use MAX in PostgreSQL to Retrieve Other Fields from a Table

Learn how to effectively use the `MAX()` function in PostgreSQL to retrieve the highest values along with other associated fields in a database table.
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This video is based on the question https://stackoverflow.com/q/74517716/ asked by the user 'aCarella' ( https://stackoverflow.com/u/2664815/ ) and on the answer https://stackoverflow.com/a/74517979/ provided by the user 'Jonathan Willcock' ( https://stackoverflow.com/u/7990032/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the Challenge of Retrieving Data with PostgreSQL's MAX Function

When working with databases, especially PostgreSQL, one common query operation is finding maximum values within specific fields. But what happens when you want to retrieve additional information alongside that maximum value? Let's take a deep dive into how to achieve this, using an example from an orders table.

Understanding the Problem

Imagine you have an orders table that records numerous transactions, and you want to find out the maximum created_date for a specific order_id, and also retrieve the id of the order entry that has this maximum date. Here's a look at the orders table structure:

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For instance, for order_id 178, you want to find:

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The Initial Attempt

A straightforward approach might be to try using the MAX() function:

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However, this results in multiple rows because you are grouping by id, which is not what we want.

The Optimal Solution

Using DISTINCT ON

To get the desired row with minimal fuss, we can employ PostgreSQL's DISTINCT ON clause. This technique simplifies finding the maximum values while allowing the inclusion of other fields. Here’s how you can structure your query:

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Breaking Down the Query:

SELECT DISTINCT ON (order_id): This tells PostgreSQL to return only the distinct order_id values.

Fields Included: We're selecting id, order_id, and created_date to get all the necessary information.

ORDER BY: The key part is ordering by order_id and then created_date in descending order. This ensures that the first record returned for each order_id is the one with the latest date.

What the Result Will Look Like

When you run the above query, PostgreSQL will return only the latest order per order_id, along with its id and created_date. For order_id 178, you will see:

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

Conclusion

Using the DISTINCT ON feature of PostgreSQL is a powerful method to retrieve maximum values along with additional fields. This approach not only meets the requirements but also enhances query efficiency and readability. So next time you need to fetch entries with maximum values from your database, remember this effective technique!

Feel free to experiment with variations of this query to adapt to your specific data retrieval needs!