Скачать или смотреть Mastering SQL: Calculate Percentage of Product Count in PostgreSQL

Learn how to calculate the `percentage` of product counts for each store using SQL in PostgreSQL. Improve your SQL skills with this easy-to-follow guide!
---
This video is based on the question https://stackoverflow.com/q/67099035/ asked by the user 'Erin L' ( https://stackoverflow.com/u/15403570/ ) and on the answer https://stackoverflow.com/a/67099058/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) 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: SQL sum and percentage

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.
---
Mastering SQL: Calculate Percentage of Product Count in PostgreSQL

When working with SQL, understanding how to perform calculations, especially for aggregating data and deriving percentages, can seem daunting for beginners. If you've ever found yourself facing a table of data, like the one below, you might have wondered how to gain more insights from it.

The Problem at Hand

Consider a table that captures the product counts for different stores:

store_idproduct_countstore150store130store220You want to know not just the total product count for each store, but also the percentage of the product count that each store represents relative to the total. The job is to convert this data into a format like this:

store_idproduct_count_sumpercentagestore180 (50 + 30)80%store22020%However, the SQL query you attempted returned a percentage of zero. Let’s explore a proper way to achieve your goal.

Understanding the Correct SQL Query

The error in your initial query likely stemmed from incorrectly aggregating the total product counts. The clear way to get both the sum and the percentage involves using window functions, which can help you calculate totals without losing the grouping context. Here's how you can do it step-by-step.

Step-by-Step Solution

Sum Product Counts: First, you need to group the data by store_id and sum the product_count for each store.

Use Window Functions: Instead of trying to get the overall sum in a correlated subquery, you can use a window function. This function allows you to calculate the total while still being within a grouped context.

Calculate Percentage: Finally, you can easily compute the percentage by dividing the counted sum by the overall sum of all product counts, multiplied by 100 to express it as a percentage.

Example Query

Here’s the SQL query that accomplishes this correctly:

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

Breakdown of the Query

SUM(product_count): This calculates the total product count for each store.

SUM(SUM(product_count)) OVER (): This uses the window function to get the grand total of the product counts across all rows.

Multiplying by 1.0: This ensures that the division returns a decimal (ratio) instead of an integer, which is crucial for accurate percentages.

Final Output: If you want to convert the ratio to a percentage, you can multiply by 100 in the selection.

Conclusion

By understanding how to leverage SQL window functions, you can accurately aggregate your data and derive meaningful insights such as percentages. This knowledge not only enhances your SQL proficiency but also allows for better data analysis in PostgreSQL. Now you're equipped to tackle similar tables with confidence and clarity! Happy querying!