How to Query DISTINCT Rows in PostgreSQL with Different ORDER BY Criteria?

PostgreSQL DISTINCT Queries: Handling ORDER BY Discrepancies

PostgreSQL's DISTINCT ON clause can present challenges when the DISTINCT ON expressions don't align with the initial ORDER BY criteria. This is because DISTINCT ON selects the first row of each group defined by the DISTINCT ON expression, as determined by the ORDER BY clause. Mismatched ordering leads to errors.

To correct this, the DISTINCT ON expression(s) should typically be the leading criteria in the ORDER BY clause. However, if a different ordering is preferred, alternative approaches are available.

Alternative Methods for Unique Row Selection

Instead of relying solely on DISTINCT ON, consider these strategies for retrieving the "top" row from each group based on a chosen ordering:

Method 1: Greatest N Per Group

This approach uses subqueries to identify the maximum (or minimum) value within each group and then joins back to the original table to retrieve the corresponding row. Here's an example:

SELECT t1.* 
FROM purchases t1
JOIN (
    SELECT address_id, max(purchased_at) as max_purchased_at
    FROM purchases
    WHERE product_id = 1
    GROUP BY address_id
) t2
ON t1.address_id = t2.address_id AND t1.purchased_at = t2.max_purchased_at
ORDER BY t1.purchased_at DESC;

Method 2: Nested DISTINCT ON Query

A more concise PostgreSQL-specific solution uses nested queries:

SELECT * 
FROM (
  SELECT DISTINCT ON (address_id) *
  FROM purchases 
  WHERE product_id = 1
  ORDER BY address_id, purchased_at DESC
) t
ORDER BY purchased_at DESC;

These methods provide flexibility in selecting unique rows while maintaining control over the final ordering, even when it differs from the grouping criteria. Choose the method that best suits your specific needs and data structure.

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