Скачать или смотреть How to Insert Elements in PostgreSQL with Maximum Row Per Person

Learn how to create a PostgreSQL insert script that assigns one file to each person in a country, ensuring no one receives more than one file at a time.
---
This video is based on the question https://stackoverflow.com/q/76878634/ asked by the user 'junior' ( https://stackoverflow.com/u/10487253/ ) and on the answer https://stackoverflow.com/a/76878822/ provided by the user 'Mike Organek' ( https://stackoverflow.com/u/13808319/ ) 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: Insert elements postgresql with maximum row

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.
---
Introduction

In this guide, we'll address a common issue when inserting data into a PostgreSQL table: how to ensure that each person in a given country receives only one document file. This is particularly important for processes where fair distribution is necessary, such as in an application that handles documents for a large number of individuals.

If you've found yourself stuck with inserting multiple files for a single individual while others go without, you're not alone. Let’s explore an effective solution that resolves this issue by distributing files evenly among people from the same country.

The Problem

You want to create an insert script for PostgreSQL that achieves the following:

Insert one file per person: Each individual should only receive a single file at a time.

Advance through individuals in a country: Once a file is assigned to one person, the next file should go to the next person from the same country.

An existing attempt resulted in multiple files being assigned to the same person, leaving others without any files. Here’s a snippet of the problematic code:

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

The Solution

To solve this problem, we need to modify the query logic to ensure that each document is only assigned to one person. This can be accomplished by using row_number() to create an ordering mechanism that helps us assign files correctly.

Step 1: Create Row Numbers

First, we establish row numbers for both the documents and the persons based on their countries. This helps us to match them appropriately.

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

Step 2: Perform a Full Join

The next step is to perform a full join. This allows us to combine both datasets based on the country and the created row numbers.

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

By using a full join, we can visualize the results and ensure the proper assignment of files to each person before actually inserting them into the history table.

Step 3: Insert into History Table

Once we have verified that the join is performing as expected, we can now modify our query to insert the results directly into the history_person_file table.

Complete Query Example

Here’s the complete insertion logic:

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

Conclusion

By applying the above method, you can efficiently manage the file assignment process in PostgreSQL, ensuring that no person receives more than one file at a time and that documents are distributed fairly among individuals in the same country.

With this structured approach, you can confidently handle similar challenges in your PostgreSQL insert scripts. Happy coding!