Скачать или смотреть How to Query MAX Column with All Related Fields in Postgres

Learn how to efficiently retrieve the maximum speed of cars along with the corresponding gear in PostgreSQL using effective SQL queries.
---
This video is based on the question https://stackoverflow.com/q/65327086/ asked by the user 'randomdude' ( https://stackoverflow.com/u/4181727/ ) and on the answer https://stackoverflow.com/a/65327145/ provided by the user 'GMB' ( https://stackoverflow.com/u/10676716/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Query associated MAX column with all it's fields in Postgres

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Solving the Problem of Retrieving Maximum Speeds with Gear Information in PostgreSQL

In the world of databases, retrieving the right information efficiently is crucial for any application. A common scenario arises when we want to query maximum values, such as the highest speeds of cars, and associate them with additional fields like gear. In this guide, we will explore a concise solution to a frequently encountered problem when using PostgreSQL, particularly how to effectively select the maximum speeds of cars and the gears in which they achieve these maximum speeds.

Understanding the Database Structure

To set the stage, let's review the database tables involved in our query:

Car: This table has an id field for each car.

Speed: This includes fields like id, actual_speed, car_id, and gear_id where:

actual_speed is the recorded speed of the car.

car_id links to the Car table.

gear_id indicates the gear at which that speed was recorded.

Gear: Contains only an id field associating a gear with a car.

Given these components, the goal is to select the maximum speeds for all cars while also identifying the gear in which those speeds were achieved.

The Initial Query Attempt

A preliminary query might look like this:

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

While this query successfully returns the maximum speeds, it fails to include the corresponding gear information. If you attempt to add gear_id to the select statement or group by clause, you may encounter errors or unwanted results. Specifically, grouping by both the car ID and gear ID causes the query to return maximum speeds for all gears instead of just the desired ones.

The Solution: Using Correlated Subqueries

To overcome this challenge, we can leverage correlated subqueries to pinpoint the maximum speeds and their associated gear for each car. Here’s how you can write the query effectively:

Correlated Subquery Approach

This solution captures the maximum speed along with its associated details:

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

In this approach:

We select all fields from the speed table where the actual_speed is equal to the maximum speed found for the given car_id.

This method benefits from potential database indexing on (car_id, actual_speed) for improved performance.

Using Distinct ON in PostgreSQL

PostgreSQL offers an efficient method using DISTINCT ON for similar queries and is particularly useful for avoiding correlated subqueries. Here’s the query using that approach:

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

In this case:

The DISTINCT ON clause ensures that only the first occurrence of each car_id is selected based on the descending order of actual_speed, thus yielding the maximum speed for each car.

Allowing for Ties with Ranking Functions

If you want to account for ties (where multiple gears might produce the same maximum speed), consider using the RANK() window function:

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

This method allows us to rank all the speeds per car and select those with a rank of 1, effectively covering any potential ties.

Conclusion

By understanding the database structure and applying these SQL techniques, you can effectively retrieve the maximum speeds of cars along with the corresponding gear information. Whether opting for a correlated subquery or the distinct approach, PostgreSQL provides robust options to handle such queries smoothly.

Utilize these methods to empower your data retrieval in PostgreSQL and ensure you can extract the most relevant information without unnecessary complications.