PostgreSQL DISTINCT ON with different ORDER BY

💡 PostgreSQL DISTINCT ON with different ORDER BY

Are you trying to use the DISTINCT ON clause in your PostgreSQL query but receiving an error? Specifically, are you encountering the error message "PG::Error: ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions"? Well, fear not! In this guide, we'll take a look at what this error means and explore some easy solutions to help you achieve your desired result without the need to order by the address_id.

🚀 Understanding the error

Let's break down the error message: "SELECT DISTINCT ON expressions must match initial ORDER BY expressions." This error occurs because PostgreSQL requires the DISTINCT ON expression to match the initial ORDER BY expression exactly. In your case, the initial ORDER BY expression is purchases.purchased_at DESC, while the DISTINCT ON expression is address_id. Since these two expressions differ, the error is triggered.

💡 Easy solution

Fortunately, there is a simple solution to this problem. Instead of directly selecting purchases.* when using DISTINCT ON, you need to include all the columns that appear in both the DISTINCT ON and ORDER BY clauses.

Here's an updated version of your query that resolves the error:

SELECT DISTINCT ON (address_id) purchases.address_id, purchases.purchased_at, purchases.column1, purchases.column2, ...
FROM purchases
WHERE purchases.product_id = 1
ORDER BY address_id, purchases.purchased_at DESC

In this updated query, we've included purchases.purchased_at in the SELECT clause, as it is part of the initial ORDER BY expression.

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Now that you know how to resolve the "SELECT DISTINCT ON expressions must match initial ORDER BY expressions" error, you can confidently utilize the DISTINCT ON clause in PostgreSQL without the need to order by the address_id. Experiment with the solution provided in this guide and see how it fits your specific use case.

If you still have any questions or need further assistance, feel free to leave a comment below. Happy coding! 😊