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88 lines
2.7 KiB
Markdown
88 lines
2.7 KiB
Markdown
---
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slug: /en/sql-reference/aggregate-functions/
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sidebar_label: Aggregate Functions
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sidebar_position: 33
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---
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# Aggregate Functions
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Aggregate functions work in the [normal](http://www.sql-tutorial.com/sql-aggregate-functions-sql-tutorial) way as expected by database experts.
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ClickHouse also supports:
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- [Parametric aggregate functions](../../sql-reference/aggregate-functions/parametric-functions.md#aggregate_functions_parametric), which accept other parameters in addition to columns.
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- [Combinators](../../sql-reference/aggregate-functions/combinators.md#aggregate_functions_combinators), which change the behavior of aggregate functions.
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## NULL Processing
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During aggregation, all `NULL`s are skipped. If the aggregation has several parameters it will ignore any row in which one or more of the parameters are NULL.
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**Examples:**
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Consider this table:
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``` text
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┌─x─┬────y─┐
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│ 1 │ 2 │
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│ 2 │ ᴺᵁᴸᴸ │
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│ 3 │ 2 │
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│ 3 │ 3 │
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│ 3 │ ᴺᵁᴸᴸ │
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└───┴──────┘
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```
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Let’s say you need to total the values in the `y` column:
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``` sql
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SELECT sum(y) FROM t_null_big
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```
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```text
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┌─sum(y)─┐
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│ 7 │
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└────────┘
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```
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Now you can use the `groupArray` function to create an array from the `y` column:
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``` sql
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SELECT groupArray(y) FROM t_null_big
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```
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``` text
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┌─groupArray(y)─┐
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│ [2,2,3] │
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└───────────────┘
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```
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`groupArray` does not include `NULL` in the resulting array.
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You can use [COALESCE](../../sql-reference/functions/functions-for-nulls.md#coalesce) to change NULL into a value that makes sense in your use case. For example: `avg(COALESCE(column, 0))` with use the column value in the aggregation or zero if NULL:
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``` sql
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SELECT
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avg(y),
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avg(coalesce(y, 0))
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FROM t_null_big
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```
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``` text
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┌─────────────avg(y)─┬─avg(coalesce(y, 0))─┐
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│ 2.3333333333333335 │ 1.4 │
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└────────────────────┴─────────────────────┘
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```
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Also you can use [Tuple](/docs/en/sql-reference/data-types/tuple.md) to work around NULL skipping behavior. The a `Tuple` that contains only a `NULL` value is not `NULL`, so the aggregate functions won't skip that row because of that `NULL` value.
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```sql
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SELECT
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groupArray(y),
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groupArray(tuple(y)).1
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FROM t_null_big;
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┌─groupArray(y)─┬─tupleElement(groupArray(tuple(y)), 1)─┐
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│ [2,2,3] │ [2,NULL,2,3,NULL] │
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└───────────────┴───────────────────────────────────────┘
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```
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