# Aggregate over window

> Supported in: Streaming

Performs the specified aggregations on the data within a window, emitting outputs as specified by the provided trigger.

**Transform categories**: Aggregate

## Declared arguments

* **Aggregate expressions:** List of aggregate expressions to evaluate over each window.<br>*List\<Expression\<AnyType>>*
* **Dataset:** Dataset to perform aggregations on.<br>*Table*
* **Window:** Window defining how elements should be grouped.<br>*Window*
* *optional* **Accumulation mode:** The accumulation mode for the window. Determines whether the window accumulates panes when the trigger fires or discards them.<br>*Enum\<Accumulating, Discarding>*
* *optional* **Key by columns:** Columns on which to partition the input by key. Each aggregation will be computed separately for each distinct key value.<br>*Set\<Column\<Binary | Boolean | Byte | Double | Float | Integer | Long | Short | String | Timestamp>>*
* *optional* **Trigger:** Trigger defining when aggregation should be performed.<br>*Trigger*