


How Can I Correctly Use PostgreSQL's DISTINCT ON with Different ORDER BY Clauses?
Understanding PostgreSQL's DISTINCT ON and ORDER BY Interactions
PostgreSQL's DISTINCT ON
clause is designed to select the first row from each group of rows that have the same values in the specified expression(s). The crucial point is that the selection of the "first" row depends entirely on the ORDER BY
clause. They must align.
A common mistake is using a DISTINCT ON
clause with an ORDER BY
clause that doesn't include the DISTINCT ON
expression(s). This leads to unpredictable results because the database's choice of the "first" row becomes arbitrary.
Correcting Order Issues with DISTINCT ON
The error arises when the fields in DISTINCT ON
don't match the leading fields in ORDER BY
. To fix this, ensure the ORDER BY
clause starts with the same expressions as DISTINCT ON
. This guarantees a consistent and predictable selection of the first row within each group.
Alternative Approaches for "Greatest N Per Group" Problems
If the objective is to find the latest purchase for each address_id
, ordered by purchase date, this is a classic "greatest N per group" query. Here are two efficient solutions:
General SQL Solution:
This approach uses a subquery to find the maximum purchased_at
for each address_id
and then joins it back to the original table to retrieve the complete row.
SELECT t1.* FROM purchases t1 JOIN ( SELECT address_id, max(purchased_at) 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
PostgreSQL-Specific Optimization:
PostgreSQL offers a more concise and potentially faster solution using a nested DISTINCT ON
query:
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 alternatives provide cleaner and more efficient solutions compared to relying solely on DISTINCT ON
when dealing with "greatest N per group" scenarios. They avoid unnecessary sorting and improve query performance.
The above is the detailed content of How Can I Correctly Use PostgreSQL's DISTINCT ON with Different ORDER BY Clauses?. For more information, please follow other related articles on the PHP Chinese website!

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