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65 lines
2.5 KiB
Markdown
65 lines
2.5 KiB
Markdown
# AggregateFunction(name, types_of_arguments...) {#data_type-aggregatefunction}
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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.
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`AggregateFunction` — parametric data type.
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**Parameters**
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- Name of the aggregate function.
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If the function is parametric specify its parameters too.
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- Types of the aggregate function arguments.
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**Example**
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```sql
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CREATE TABLE t
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(
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column1 AggregateFunction(uniq, UInt64),
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column2 AggregateFunction(anyIf, String, UInt8),
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column3 AggregateFunction(quantiles(0.5, 0.9), UInt64)
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) ENGINE = ...
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```
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[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-combinator-if)) and [quantiles](../../query_language/agg_functions/reference.md) are the aggregate functions supported in ClickHouse.
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## Usage
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### Data Insertion
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To insert data, use `INSERT SELECT` with aggregate `-State`- functions.
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**Function examples**
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```
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uniqState(UserID)
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quantilesState(0.5, 0.9)(SendTiming)
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```
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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.
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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.
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### Data Selection
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When selecting data from `AggregatingMergeTree` table, use `GROUP BY` clause and the same aggregate functions as when inserting data, but using `-Merge`suffix.
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An aggregate function with `-Merge` suffix takes a set of states, combines them, and returns the result of complete data aggregation.
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For example, the following two queries return the same result:
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```sql
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SELECT uniq(UserID) FROM table
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SELECT uniqMerge(state) FROM (SELECT uniqState(UserID) AS state FROM table GROUP BY RegionID)
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```
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## Usage Example
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See [AggregatingMergeTree](../../operations/table_engines/aggregatingmergetree.md) engine description.
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[Original article](https://clickhouse.yandex/docs/en/data_types/nested_data_structures/aggregatefunction/) <!--hide-->
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