How to Resolve PostgreSQL's 'SELECT DISTINCT ON expressions must match initial ORDER BY expressions' Error?

PostgreSQL DISTINCT ON Error: Mismatched ORDER BY Clauses

Using PostgreSQL's DISTINCT ON with an ORDER BY clause containing different expressions can lead to an error: "SELECT DISTINCT ON expressions must match initial ORDER BY expressions." This occurs because DISTINCT ON selects unique rows based on specified columns, while ORDER BY dictates the presentation order of those unique rows. The columns in DISTINCT ON must therefore match the leading columns in the ORDER BY clause.

For instance, if you need to retrieve distinct address_id values while prioritizing rows with the latest purchased_at timestamp, simply sorting by address_id in the ORDER BY clause isn't sufficient when using DISTINCT ON (address_id). This is because DISTINCT ON will select the first row for each address_id encountered according to the ORDER BY clause.

Here are two effective workarounds:

Method 1: Subquery for Maximum purchased_at

This approach uses a subquery to find the maximum purchased_at for each address_id, then joins this result back to the original table to select the corresponding rows:

<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 with DISTINCT ON and ORDER BY

This PostgreSQL-specific solution uses a nested query. The inner query uses DISTINCT ON to select the desired unique rows, ordered appropriately, and the outer query reorders the results:

<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>

Both methods provide efficient and accurate results, overcoming the limitations imposed by directly combining DISTINCT ON and mismatched ORDER BY clauses. Choose the method that best suits your coding style and database performance requirements.

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