> ## Documentation Index
> Fetch the complete documentation index at: https://alphacastio.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Regroup Entities

> Drop an entity column and aggregate the remaining rows with a formula — the equivalent of GROUP BY.

The **Regroup Entities** step removes an entity and regroups the data by the ones that remain, using a formula you choose to aggregate. It is the equivalent of **GROUP BY** in SQL or `groupby` in pandas.

## Why entities need care

Entity columns are the columns required to **uniquely** identify a row. Because of that, a combination of entity values can never repeat — which makes removing or changing entities a delicate operation that can corrupt the data if done carelessly.

Say you have a dataset with Date and Country as entities, the most common combination in Alphacast. You will have many rows for the same date, one per country. **You cannot simply drop the Country column**, because dates would then repeat and entity combinations must be unique.

<Frame>
  <img src="https://mintcdn.com/alphacastio/ttF_U-V9HDrmrPbo/images/steps/regroup-entities-1.png?fit=max&auto=format&n=ttF_U-V9HDrmrPbo&q=85&s=f557f47ac3184d5d195ac68cfe89b0c9" alt="A dataset with Date and Country as entities" width="902" height="335" data-path="images/steps/regroup-entities-1.png" />
</Frame>

Regrouping is how you resolve this.

## How to regroup

<Steps>
  <Step title="Create a pipeline and select the source">
    Start from a [Fetch Dataset](/steps/fetch-dataset) step.
  </Step>

  <Step title="Add the Regroup Entities step" />

  <Step title="Choose which entities to drop">
    Deselect the entities you want to remove.
  </Step>

  <Step title="Choose the aggregation per column">
    Decide how rows that now share the same entity values should be combined — the Date, in the example above.

    The aggregation is set **per column**, so different columns can be combined differently in the same step. The right choice depends entirely on what each column represents.

    | Aggregation      | What it does                                    |
    | ---------------- | ----------------------------------------------- |
    | `sum`            | Total of the group's values.                    |
    | `mean` / `avg`   | Arithmetic mean, ignoring nulls.                |
    | `median`         | Median, ignoring nulls.                         |
    | `min` / `max`    | Smallest or largest value in the group.         |
    | `count`          | Number of non-zero values.                      |
    | `product`        | Product of the group's values.                  |
    | `std` / `var`    | Standard deviation or variance, ignoring nulls. |
    | `first` / `last` | First or last value in the group.               |
    | `meanw`          | Weighted mean. Requires a weight column.        |
    | `sumw`           | Weighted sum. Requires a weight column.         |
    | `drop`           | Discards the column instead of aggregating it.  |

    <Tip>
      Weighted aggregations are the right choice when combining rates or ratios across entities — a weighted mean of inflation across countries needs a population or GDP weight column, since a plain mean would treat every country equally.
    </Tip>
  </Step>
</Steps>

The result keeps every entity except the ones you excluded, and has fewer rows than the original because rows sharing entity values are collapsed together.

<Frame>
  <img src="https://mintcdn.com/alphacastio/ttF_U-V9HDrmrPbo/images/steps/regroup-entities-2.png?fit=max&auto=format&n=ttF_U-V9HDrmrPbo&q=85&s=37f091ecab422dadb05143aa202a0b16" alt="Result of regrouping entities" width="555" height="523" data-path="images/steps/regroup-entities-2.png" />
</Frame>

<Note>
  This step uses the fixed list of aggregations above, not the formula editor. To compute a value that isn't in the list, add a [Calculate Variable](/steps/calculate-variable) step before or after regrouping.
</Note>

## Next steps

<CardGroup cols={2}>
  <Card title="Calculate variable" icon="function" href="/steps/calculate-variable">
    Compute values the fixed aggregations don't cover.
  </Card>

  <Card title="Change frequency" icon="clock" href="/steps/resample">
    Aggregate along the time dimension instead.
  </Card>
</CardGroup>