How to Resolve PostgreSQL's DISTINCT ON Mismatched ORDER BY Clause Error?

Troubleshooting PostgreSQL's DISTINCT ON Clause Mismatch Error

PostgreSQL's DISTINCT ON clause requires the ORDER BY expression to precisely match the distinct columns. Attempting a query with differing expressions will result in an error. Simply adding the address_id as the first element in the ORDER BY clause, while a solution to the error, might not return the intended results.

Here are alternative methods to retrieve the correct data without altering the ORDER BY within the DISTINCT ON clause:

Method 1: Greatest-N-Per-Group Approach (DBMS-Agnostic)

This method uses a subquery to find the maximum purchased_at for each address_id, then joins back to the original table to retrieve the complete row:

<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: PostgreSQL-Specific Solution

This approach leverages a nested query, using DISTINCT ON in the inner query to select the desired row for each address_id, and then ordering the final results as needed in the outer query:

<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 approaches provide flexible solutions for retrieving the necessary data while maintaining the desired sorting order, avoiding the need to compromise on the DISTINCT ON clause's ORDER BY expression.

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