Скачать или смотреть How to Use GROUP BY in PostgreSQL for Multiple Selections

Discover how to efficiently use `GROUP BY` and `DISTINCT ON` in PostgreSQL to retrieve the minimum values from a table while keeping track of unique IDs.
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This video is based on the question https://stackoverflow.com/q/68714331/ asked by the user 'user52219' ( https://stackoverflow.com/u/9123522/ ) and on the answer https://stackoverflow.com/a/68714363/ 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.

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Understanding PostgreSQL's Grouping and Selection

When working with databases, particularly PostgreSQL, you may find yourself needing to retrieve specific rows based on certain conditions. One common scenario is selecting a minimum value from a column while keeping track of other associated data, like an ID. In this post, we will explore a practical example of how to transform a table while maintaining unique entries based on a group of categories.

The Problem

Imagine you have a table structured as follows:

idvaluecategory110A290A380B420BYou want to transform this table so that for each category, you keep only the entry with the minimum value, as well as the corresponding ID. The expected output should look like this:

idvaluecategory110A420BThis provides a clear way to view the lowest values per category along with their respective IDs.

The Initial Attempt

Your initial SQL query might look something like this:

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

While this query correctly groups by category, it won't return the associated ID for the minimum value, leading to incorrect or incomplete results.

The Solution: Using DISTINCT ON

To successfully achieve your goal in PostgreSQL, you can utilize the DISTINCT ON clause. This approach allows you to select unique entries based on a particular column or criteria while retrieving the full row data.

Here’s How You Can Do It:

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

Breakdown of the Query:

SELECT DISTINCT ON (category): This tells PostgreSQL to provide distinct rows based on the category column.

t.*: The asterisk indicates that you're selecting all columns from the table (in this case, id, value, and category).

FROM t: Here, t is the name of your table. Replace it with the actual table name as necessary.

ORDER BY category, value: This ensures that the result is ordered first by category and then by value. The database will pick the first occurrence of each category, which becomes the row with the minimum value due to the order specified.

Conclusion

With this method, you can quickly and efficiently retrieve the minimum values from your PostgreSQL table while maintaining important associated data like IDs. The use of DISTINCT ON makes it easy to achieve this without complex subqueries or joins.

If you often work with grouped data and require minimum values, incorporating this approach into your SQL toolkit will save you time and lead to clearer data presentations. Happy querying!