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5.0 KiB
Markdown
137 lines
5.0 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` arguments are skipped. If the aggregation has several arguments it will ignore any row in which one or more of them are NULL.
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There is an exception to this rule, which are the functions [`first_value`](../../sql-reference/aggregate-functions/reference/first_value.md), [`last_value`](../../sql-reference/aggregate-functions/reference/last_value.md) and their aliases when followed by the modifier `RESPECT NULLS`: `FIRST_VALUE(b) RESPECT NULLS`.
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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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Note that aggregations are skipped when the columns are used as arguments to an aggregated function. For example [`count`](../../sql-reference/aggregate-functions/reference/count.md) without parameters (`count()`) or with constant ones (`count(1)`) will count all rows in the block (independently of the value of the GROUP BY column as it's not an argument), while `count(column)` will only return the number of rows where column is not NULL.
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```sql
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SELECT
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v,
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count(1),
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count(v)
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FROM
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(
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SELECT if(number < 10, NULL, number % 3) AS v
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FROM numbers(15)
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)
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GROUP BY v
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┌────v─┬─count()─┬─count(v)─┐
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│ ᴺᵁᴸᴸ │ 10 │ 0 │
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│ 0 │ 1 │ 1 │
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│ 1 │ 2 │ 2 │
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│ 2 │ 2 │ 2 │
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└──────┴─────────┴──────────┘
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```
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And here is an example of of first_value with `RESPECT NULLS` where we can see that NULL inputs are respected and it will return the first value read, whether it's NULL or not:
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```sql
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SELECT
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col || '_' || ((col + 1) * 5 - 1) as range,
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first_value(odd_or_null) as first,
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first_value(odd_or_null) IGNORE NULLS as first_ignore_null,
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first_value(odd_or_null) RESPECT NULLS as first_respect_nulls
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FROM
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(
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SELECT
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intDiv(number, 5) AS col,
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if(number % 2 == 0, NULL, number) as odd_or_null
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FROM numbers(15)
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)
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GROUP BY col
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ORDER BY col
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┌─range─┬─first─┬─first_ignore_null─┬─first_respect_nulls─┐
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│ 0_4 │ 1 │ 1 │ ᴺᵁᴸᴸ │
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│ 1_9 │ 5 │ 5 │ 5 │
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│ 2_14 │ 11 │ 11 │ ᴺᵁᴸᴸ │
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└───────┴───────┴───────────────────┴─────────────────────┘
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```
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