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Скачать или смотреть ⚡ SQL One-Liner: Calculate Running Average Instantly!

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  • 2025-03-22
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⚡ SQL One-Liner: Calculate Running Average Instantly!
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Описание к видео ⚡ SQL One-Liner: Calculate Running Average Instantly!

In this solution, we demonstrate two approaches to calculate a running average in SQL. A running average helps you analyze trends over time by calculating the cumulative average of a measure (for example, sales or salaries) up to each row in your dataset. Below, we explain each step and the techniques used:

1. Traditional Approach Using a Correlated Subquery:

SELECT t1.ID,
t1.Value,
(SELECT AVG(t2.Value)
FROM YourTable t2
WHERE t2.ID v= t1.ID) AS RunningAverage
FROM YourTable t1
ORDER BY t1.ID;

Step-by-Step Explanation:

Table Alias & Ordering:

We alias the table as t1 in the outer query.

The query is ordered by ID, which should be a sequential column (or a timestamp) ensuring that earlier rows come first.

Correlated Subquery:

Inside the SELECT list, we include a subquery that calculates the average (AVG) of the Value column.

This subquery is correlated because it references t1.ID from the outer query. For each row in t1, the subquery finds all rows in YourTable (aliased as t2) with an ID less than or equal to the current t1.ID.

Running Average Calculation:

The subquery returns the average of the Value from the beginning of the dataset up to the current row. This result is labeled as RunningAverage.

Key Concepts:

Correlated Subquery: A subquery that depends on a value from the outer query. It is executed repeatedly—once for each row processed by the outer query.

Aggregate Function (AVG): Calculates the average of the numeric values in a specified column.

2. Shortcut One-Liner Using a Window Function:

SELECT ID,
Value,
AVG(Value) OVER (ORDER BY ID) AS RunningAverage
FROM YourTable
ORDER BY ID;

Step-by-Step Explanation:

Window Function Basics:

A window function performs a calculation across a set of table rows that are somehow related to the current row.

The syntax AVG(Value) OVER (ORDER BY ID) computes the running average for each row based on all rows preceding and including the current row, ordered by ID.

Ordering with OVER Clause:

The ORDER BY ID within the OVER clause tells SQL how to define the window frame (i.e., which rows to include for each calculation).

This ensures that for each row, the function calculates the average of Value from the start of the dataset up to that row.

Efficiency and Readability:

Using the window function, the running average is computed in one pass over the data, making it much more efficient and the query significantly cleaner compared to using a correlated subquery.

Key Concepts:

Window Function: A function that performs a calculation across a set of rows relative to the current row without collapsing the rows into a single result (unlike aggregate functions used with GROUP BY).

OVER Clause: Defines the window (set of rows) for the window function. It can include ORDER BY, which determines the order in which the rows are processed.

AVG() Window Function: Computes the average for each window frame specified by the OVER clause.

Summary:

Traditional Method:

Uses a correlated subquery to compute the running average. While straightforward, it can be less efficient as the subquery is executed for each row.

Shortcut One-Liner:

Utilizes a window function (AVG() OVER (ORDER BY ID)) to calculate the running average in a single, efficient query. This modern method reduces complexity and improves performance.

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