How to Correctly Use PostgreSQL Window Functions with GROUP BY Clauses?

PostgreSQL Window Function and Group By Exception

When attempting to use a window function in conjunction with a GROUP BY clause in PostgreSQL, it's crucial to understand the distinction between the two. Window functions operate on a partition of table data, performing calculations across rows while preserving all rows, unlike aggregate functions that summarize rows into a single value. Therefore, when using window functions, it's essential to include the partitioned columns in the GROUP BY clause.

Issue Description

In the provided query, the user encounters an error when attempting to execute the following code:

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

The error arises because the window functions, sum(sp.payout) OVER (ORDER BY e.date) and sum(s.buyin) OVER (ORDER BY e.date), are not accompanied by an aggregate function. As a result, PostgreSQL demands that the partitioned columns, sp.payout and s.buyin, be included in the GROUP BY clause.

Solution

To address the issue, the user can either:

1. Include sp.payout and s.buyin in the GROUP BY clause:

GROUP BY p.name, e.date, e.event_id

However, this approach may lead to multiple rows for each player and event if there are multiple sp.payout or s.buyin values.

2. Employ the combination of window functions and aggregate functions:

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;

In this example, the sum(sum(sp.payout)) OVER w and sum(sum(s.buyin)) OVER w expressions combine an outer window function with an inner aggregate function to calculate the total payouts and buyins per event.

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