How does window functions work internally in MySQL?

Window functions in MySQL allow users to perform calculations across sets of rows related to the current row. Unlike aggregate functions that group rows into a single output, window functions retain the individual row identities while allowing for computation across a defined range of rows. This functionality is vital for analytical queries where you need insights on data without losing the detail of individual records.

The internal workings of window functions involve creating a virtual table that represents a subset of rows based on the specified partition and order criteria. Each row in the result has access to values in other rows of the window, making it possible to perform calculations like running totals, moving averages, rankings, and more.

When a query using window functions is executed, MySQL first processes the windowing specification, which defines how rows are to be partitioned and ordered. After the partitions are established, the window functions are applied across those partitions, allowing for flexible and sophisticated analyses.

An example of a window function in MySQL:

SELECT employee_id, salary, AVG(salary) OVER (PARTITION BY department_id) AS avg_department_salary FROM employees;

window functions MySQL SQL analytics data analysis partitioning running total moving averages

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