How to Resolve 'DISTINCT ON Expressions Must Match Initial ORDER BY Expressions' in PostgreSQL?

PostgreSQL's DISTINCT ON clause simplifies retrieving the first row for each unique value within a set. However, a common pitfall arises when the DISTINCT ON expression doesn't align with the initial ORDER BY expression.

DISTINCT ON Expression Mismatch Error

This error frequently occurs:

<code class="language-sql">SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM purchases
WHERE purchases.product_id = 1
ORDER BY purchases.purchased_at DESC</code>

Resulting in:

<code>PG::Error: ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions</code>

The Solution: Aligning DISTINCT ON and ORDER BY

PostgreSQL's documentation mandates that the DISTINCT ON expression(s) must mirror the leftmost ORDER BY expression(s). The solution is straightforward: reorder your ORDER BY clause:

<code class="language-sql">SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM purchases
WHERE purchases.product_id = 1
ORDER BY address_id, purchases.purchased_at DESC</code>

Alternative Methods: Bypassing address_id Ordering

If you need to avoid ordering by address_id, consider these alternatives:

Method 1: The "Greatest N per group" Approach

This method efficiently finds the most recent purchase for each address_id:

<code class="language-sql">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</code>

Method 2: Nested Query for PostGreSQL

This approach uses a subquery to achieve the desired result while maintaining the purchased_at ordering:

<code class="language-sql">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</code>

These alternatives provide flexibility when you don't want to prioritize address_id in the final output ordering. They effectively address the "most recent purchase per address" problem without directly violating the DISTINCT ON constraint.

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