Why Does My MySQL GROUP BY Query Fail in PostgreSQL?

PostgreSQL and MySQL: A GROUP BY Clause Comparison

Migrating MySQL queries to PostgreSQL can lead to the error "column 'XYZ' must appear in the GROUP BY clause or be used in an aggregate function." This discrepancy stems from how these databases handle GROUP BY clauses.

MySQL's Non-Standard Approach

MySQL's GROUP BY implementation deviates from the SQL standard. It permits selecting columns not explicitly listed in the GROUP BY clause, resulting in unpredictable output. The selected row from each group is determined implicitly, influenced by factors like indexing and query optimization.

PostgreSQL's Standard-Compliant GROUP BY

PostgreSQL, however, strictly adheres to the SQL standard. All columns in the SELECT list must either be included in the GROUP BY clause or be part of an aggregate function (like SUM, AVG, COUNT). Otherwise, the aforementioned error occurs.

Replicating MySQL's Behavior in PostgreSQL

While PostgreSQL's approach is more predictable and standards-compliant, you can simulate MySQL's non-standard behavior using DISTINCT ON. The following PostgreSQL query demonstrates this:

SELECT DISTINCT ON (availables.bookdate)
availables.*,
rooms.hotel_id
FROM availables
INNER JOIN rooms ON rooms.id = availables.room_id
WHERE rooms.hotel_id = 5056
AND availables.bookdate BETWEEN '2009-11-22' AND '2009-11-24'
ORDER BY availables.updated_at;

DISTINCT ON returns a single row for each distinct value in availables.bookdate, maintaining the row order specified by the ORDER BY clause. This effectively mimics MySQL's non-standard GROUP BY behavior in this specific scenario. Note that this approach relies on the ORDER BY clause to define which row is chosen for each distinct group.

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