ClickHouse/docs/en/data_types/nested_data_structures/aggregatefunction.md

<a name="data_type-aggregatefunction"></a>

# AggregateFunction(name, types_of_arguments...)

The intermediate state of an aggregate function. To get it, use aggregate functions with the `-State` suffix. To get aggregated data in the future, you must use the same aggregate functions with the `-Merge`suffix.

`AggregateFunction` — parametric data type.

**Parameters**

- Name of the aggregate function.

    If the function is parametric specify its parameters too.

- Types of the aggregate function arguments.

**Example**

```sql
CREATE TABLE t
(
    column1 AggregateFunction(uniq, UInt64),
    column2 AggregateFunction(anyIf, String, UInt8),
    column3 AggregateFunction(quantiles(0.5, 0.9), UInt64)
) ENGINE = ...
```

[uniq](../../query_language/agg_functions/reference.md#agg_function-uniq), anyIf ([any](../../query_language/agg_functions/reference.md#agg_function-any)+[If](../../query_language/agg_functions/combinators.md#agg-functions-combine-if)) and [quantiles](../../query_language/agg_functions/reference.md#agg_function-quantiles) are the aggregate functions supported in ClickHouse.

## Usage

### Data Insertion

To insert data, use `INSERT SELECT` with aggregate `-State`- functions.

**Function examples**

```
uniqState(UserID)
quantilesState(0.5, 0.9)(SendTiming)
```

In contrast to the corresponding functions `uniq` and `quantiles`, `-State`- functions return the state, instead the final value. In other words, they return a value of `AggregateFunction` type.

In the results of `SELECT` query the values of  `AggregateFunction` type have implementation-specific binary representation for all of the ClickHouse output formats. If dump data into, for example, `TabSeparated` format with `SELECT` query then this dump can be loaded back using `INSERT` query.

### Data Selection

When selecting data from `AggregatingMergeTree` table, use `GROUP BY` clause and the same aggregate functions as when inserting data, but using `-Merge`suffix.

An aggregate function with `-Merge` suffix takes a set of states, combines them, and returns the result of complete data aggregation.

For example, the following two queries return the same result:

```sql
SELECT uniq(UserID) FROM table

SELECT uniqMerge(state) FROM (SELECT uniqState(UserID) AS state FROM table GROUP BY RegionID)
```

## Usage Example

See [AggregatingMergeTree](../../operations/table_engines/aggregatingmergetree.md#table_engine-aggregatingmergetree) engine description.


[Original article](https://clickhouse.yandex/docs/en/data_types/nested_data_structures/aggregatefunction/) <!--hide-->