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Description

This `42P20` error in PostgreSQL indicates a problem with the syntax or usage of a window function in your SQL query. It typically occurs when the `OVER` clause or its partitions/ordering are incorrectly defined, or when certain functions are used inappropriately within a window context, preventing the query from executing.

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Error Message

windowing error

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Known Causes

3 known causes

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Invalid Window Function Syntax

The most common cause is a mistake in the `OVER` clause, such as missing parentheses, incorrect partition specifications, or invalid ordering expressions.

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Improper Function Usage

Attempting to use an aggregate function as a window function without an `OVER` clause, or using a window function in an unsupported context (e.g., within `WHERE` or `GROUP BY`).

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Conflicting Query Clauses

This error can arise from combining window functions with other SQL clauses (like `GROUP BY` or `DISTINCT`) in a way that creates an ambiguous or invalid query structure.

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Solutions

4 solutions available

1. Correct `OVER` Clause Syntax easy

Ensure the `OVER` clause is correctly structured with `PARTITION BY` and `ORDER BY` clauses.

1

Review the `OVER` clause in your SQL query. Window functions require an `OVER` clause to define the window of rows the function operates on.

-- Incorrect syntax example:
SELECT
    column1,
    SUM(column2) OVER (PARTITION BY column3)
FROM
    your_table;

-- Correct syntax example:
SELECT
    column1,
    SUM(column2) OVER (PARTITION BY column3 ORDER BY column1 ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
FROM
    your_table;

2

Verify that `PARTITION BY` and `ORDER BY` are used appropriately. `PARTITION BY` divides rows into partitions, and `ORDER BY` defines the order within each partition. If you need to specify a frame, ensure it's also correctly defined (e.g., `ROWS BETWEEN ...`).

-- Example with explicit frame definition:
SELECT
    product_name,
    sale_date,
    sale_amount,
    AVG(sale_amount) OVER (
        PARTITION BY product_name
        ORDER BY sale_date
        ROWS BETWEEN 2 PRECEDING AND 2 FOLLOWING
    ) AS rolling_avg_sales
FROM
    sales_data;

2. Validate Window Function Arguments medium

Confirm that the arguments provided to the window function are valid and compatible.

1

Check the specific window function being used (e.g., `ROW_NUMBER()`, `RANK()`, `SUM()`, `AVG()`, `LAG()`, `LEAD()`). Ensure that the data types of the columns used as arguments within the window function are compatible with the function's requirements.

-- Example: LAG requires a column and an optional offset and default value.
-- If column3 is not a numeric type, SUM() might fail.
SELECT
    column1,
    column2,
    LAG(column2, 1, 0) OVER (PARTITION BY column1 ORDER BY column3) AS previous_value
FROM
    your_table;

2

Some window functions, like `FIRST_VALUE()` or `LAST_VALUE()`, require an `ORDER BY` clause within the `OVER` clause to function correctly. Ensure it's present if needed.

-- Example where ORDER BY is crucial for FIRST_VALUE:
SELECT
    category,
    product_id,
    price,
    FIRST_VALUE(product_id) OVER (
        PARTITION BY category
        ORDER BY price DESC
    ) AS most_expensive_product_id
FROM
    products;

3. Check for Missing `ORDER BY` in Frame Definitions medium

Ensure an `ORDER BY` clause exists within the `OVER` clause when defining row frames.

1

When you explicitly define a row frame (e.g., `ROWS BETWEEN ...` or `RANGE BETWEEN ...`), PostgreSQL requires an `ORDER BY` clause within the `OVER` clause to determine the order of rows for frame calculation.

-- Incorrect: No ORDER BY for frame definition
SELECT
    event_time,
    value,
    AVG(value) OVER (
        PARTITION BY user_id
        ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
    )
FROM
    user_events;

-- Correct: ORDER BY added
SELECT
    event_time,
    value,
    AVG(value) OVER (
        PARTITION BY user_id
        ORDER BY event_time
        ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
    )
FROM
    user_events;

4. Simplify and Isolate the Window Function medium

Break down complex queries to pinpoint the problematic window function or clause.

1

If you have a complex query with multiple window functions or subqueries, try to isolate the problematic window function. Comment out other parts of the query or create a simplified version.

-- Original complex query (example):
SELECT
    a.col1,
    b.col2,
    SUM(a.col3) OVER (PARTITION BY a.col1 ORDER BY a.col4) AS window_sum,
    ROW_NUMBER() OVER (PARTITION BY b.col2 ORDER BY a.col4 DESC) AS rn
FROM
    table_a a
JOIN
    table_b b ON a.id = b.id
WHERE
    a.col5 > 10;

2

Execute the query with only one window function at a time to see which one triggers the error. Gradually add complexity back until the error reappears.

-- Simplified query to test SUM window function:
SELECT
    a.col1,
    SUM(a.col3) OVER (PARTITION BY a.col1 ORDER BY a.col4) AS window_sum
FROM
    table_a a
WHERE
    a.col5 > 10;

-- Simplified query to test ROW_NUMBER window function:
SELECT
    b.col2,
    ROW_NUMBER() OVER (PARTITION BY b.col2 ORDER BY a.col4 DESC) AS rn
FROM
    table_a a
JOIN
    table_b b ON a.id = b.id
WHERE
    a.col5 > 10;

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Related Errors

5 related errors

PostgreSQL Error 42P20: Window Function Syntax Error