ClickHouse/docs/zh/sql-reference/aggregate-functions/reference/initializeAggregation.md

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initializeAggregation

Initializes aggregation for your input rows. It is intended for the functions with the suffix State. Use it for tests or to process columns of types AggregateFunction and AggregationgMergeTree.

Syntax

initializeAggregation (aggregate_function, column_1, column_2);

Parameters

• aggregate_function — Name of the aggregation function. The state of this function — the creating one. String.
• column_n — The column to translate it into the function as it's argument. String.

Returned value(s)

Returns the result of the aggregation for your input rows. The return type will be the same as the return type of function, that initializeAgregation takes as first argument. For example for functions with the suffix State the return type will be AggregateFunction.

Example

Query:

SELECT uniqMerge(state) FROM (SELECT initializeAggregation('uniqState', number % 3) AS state FROM system.numbers LIMIT 10000);

Result:

┌─uniqMerge(state)─┐ │ 3 │ └──────────────────┘