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Window Functions
================

In this section you will learn about window functions. A window function utilizes values from one or multiple rows to
produce a result for each individual row, unlike an aggregate function that provides a single value for multiple rows.

The functionality of window functions in DataFusion is supported by the dedicated :func:`.window` function.

We'll use the pokemon dataset (from Ritchie Vink) in the following examples.

.. ipython:: python

    import urllib.request
    from datafusion import SessionContext
    from datafusion import col
    from datafusion import functions as f

    urllib.request.urlretrieve(
        "https://gist.githubusercontent.com/ritchie46/cac6b337ea52281aa23c049250a4ff03/raw/89a957ff3919d90e6ef2d34235e6bf22304f3366/pokemon.csv",
        "pokemon.csv",
    )

    ctx = SessionContext()
    df = ctx.read_csv("pokemon.csv")

Here is an example that shows how to compare each pokemons’s attack power with the average attack power in its :code:`"Type 1"`

.. ipython:: python

    df.select(
        col('"Name"'),
        col('"Attack"'),
        f.alias(
            f.window("avg", [col('"Attack"')], partition_by=[col('"Type 1"')]),
            "Average Attack",
        )
    )

You can also control the order in which rows are processed by window functions by providing
a list of :func:`.order_by` functions for the :code:`order_by` parameter.

.. ipython:: python

    df.select(
        col('"Name"'),
        col('"Attack"'),
        f.alias(
            f.window(
                "rank",
                [],
                partition_by=[col('"Type 1"')],
                order_by=[f.order_by(col('"Attack"'))],
            ),
            "rank",
        ),
    )

The possible window functions are:

1. Rank Functions
    - rank
    - dense_rank
    - row_number
    - ntile

2. Analytical Functions
    - cume_dist
    - percent_rank
    - lag
    - lead
    - first_value
    - last_value
    - nth_value

3. Aggregate Functions
    - All aggregate functions can be used as window functions.