Why Does My PostgreSQL Query Generate a 'column 'sp.payout' must appear in the GROUP BY clause' Error When Using Window Functions?

Window Function and Group By Exception in PostgreSQL

In this query, the goal is to calculate the cumulative profit/loss for a specific user over time. However, the initial attempt encounters an error due to a misunderstanding about the nature of window functions in PostgreSQL.

Misidentified Aggregate Functions

The error message "column "sp.payout" must appear in the GROUP BY clause or be used in an aggregate function" indicates that PostgreSQL expects the columns sp.payout and s.buyin to be included in the GROUP BY clause because they are used in the purported aggregate function sum(). However, this is a misidentification.

Utilizing Window Functions

In this query, sum() is being used as a window function, not an aggregate function. Window functions, unlike aggregate functions, aggregate values within a specified range, but they retain all individual rows after the operation. This is a significant difference that the query's incorrect assumption about aggregate functions overlooks.

Combining Window and Aggregate Functions

The solution is to correctly identify and utilize window functions. PostgreSQL allows for the combination of window and aggregate functions, with aggregate functions being applied first.

Revised Query

The revised query incorporates this understanding:

SELECT p.name
     , e.event_id
     , e.date
     , sum(sum(sp.payout)) OVER w
     - sum(sum(s.buyin  )) OVER w AS "Profit/Loss" 
FROM   player            p
JOIN   result            r ON r.player_id     = p.player_id  
JOIN   game              g ON g.game_id       = r.game_id 
JOIN   event             e ON e.event_id      = g.event_id 
JOIN   structure         s ON s.structure_id  = g.structure_id 
JOIN   structure_payout sp ON sp.structure_id = g.structure_id
                          AND sp.position     = r.position
WHERE  p.player_id = 17 
GROUP  BY e.event_id
WINDOW w AS (ORDER BY e.date, e.event_id)
ORDER  BY e.date, e.event_id;

Explanation

The outer sum() in this expression (sum(sum(sp.payout)) OVER w) is a window function, while the inner sum() is an aggregate function. The aggregate function sums the payout and buyin values within each event, and the window function then sums these aggregate results over the specified range.

Key Points

• Window functions aggregate values over a specified range, keeping all rows after the operation.
• Aggregate functions are applied first when combining window and aggregate functions.
• Using the correct function type (window or aggregate) is crucial for proper query execution.

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