mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-18 05:32:52 +00:00
62 lines
1.4 KiB
Markdown
62 lines
1.4 KiB
Markdown
---
|
||
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](../../sql-reference/aggregate-functions/parametric-functions.md#aggregate_functions_parametric), which accept other parameters in addition to columns.
|
||
- [Combinators](../../sql-reference/aggregate-functions/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 │
|
||
└───┴──────┘
|
||
```
|
||
|
||
Let’s say you need to total the values in the `y` column:
|
||
|
||
``` sql
|
||
SELECT sum(y) FROM t_null_big
|
||
```
|
||
|
||
```text
|
||
┌─sum(y)─┐
|
||
│ 7 │
|
||
└────────┘
|
||
```
|
||
|
||
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.
|
||
|
||
|