mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-15 03:53:41 +00:00
75 lines
2.1 KiB
Markdown
75 lines
2.1 KiB
Markdown
---
|
||
slug: /en/sql-reference/aggregate-functions/
|
||
sidebar_label: Aggregate Functions
|
||
sidebar_position: 33
|
||
---
|
||
|
||
# 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
|
||
|
||
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.
|
||
|
||
**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.
|
||
|
||
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:
|
||
|
||
``` sql
|
||
SELECT
|
||
avg(y),
|
||
avg(coalesce(y, 0))
|
||
FROM t_null_big
|
||
```
|
||
|
||
``` text
|
||
┌─────────────avg(y)─┬─avg(coalesce(y, 0))─┐
|
||
│ 2.3333333333333335 │ 1.4 │
|
||
└────────────────────┴─────────────────────┘
|
||
```
|