ClickHouse/docs/en/sql-reference/aggregate-functions/index.md
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---
toc_folder_title: Aggregate Functions
toc_priority: 33
toc_title: Introduction
---
# Aggregate Functions {#aggregate-functions}
Aggregate functions work in the [normal](http://www.sql-tutorial.com/sql-aggregate-functions-sql-tutorial) way as expected by database experts.
ClickHouse also supports:
- [Parametric aggregate functions](parametric-functions.md#aggregate_functions_parametric), which accept other parameters in addition to columns.
- [Combinators](combinators.md#aggregate_functions_combinators), which change the behavior of aggregate functions.
## NULL Processing {#null-processing}
During aggregation, all `NULL`s are skipped.
**Examples:**
Consider this table:
``` text
┌─x─┬────y─┐
│ 1 │ 2 │
│ 2 │ ᴺᵁᴸᴸ │
│ 3 │ 2 │
│ 3 │ 3 │
│ 3 │ ᴺᵁᴸᴸ │
└───┴──────┘
```
Lets say you need to total the values in the `y` column:
``` sql
SELECT sum(y) FROM t_null_big
```
┌─sum(y)─┐
│ 7 │
└────────┘
The `sum` function interprets `NULL` as `0`. In particular, this means that if the function receives input of a selection where all the values are `NULL`, then the result will be `0`, not `NULL`.
Now you can use the `groupArray` function to create an array from the `y` column:
``` sql
SELECT groupArray(y) FROM t_null_big
```
``` text
┌─groupArray(y)─┐
│ [2,2,3] │
└───────────────┘
```
`groupArray` does not include `NULL` in the resulting array.
[Original article](https://clickhouse.tech/docs/en/query_language/agg_functions/) <!--hide-->