ClickHouse/docs/en/query_language/agg_functions/index.md
BayoNet 324fd98de6 Update of english documentation (#2918)
* Updating of english translation.

* Some bugs are fixed.
2018-09-04 14:18:59 +03:00

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<a name="aggregate_functions"></a>
# 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
During aggregation, all `NULL`s are skipped.
**Examples:**
Consider this table:
```
┌─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:
```
:) SELECT sum(y) FROM t_null_big
SELECT sum(y)
FROM t_null_big
┌─sum(y)─┐
│ 7 │
└────────┘
1 rows in set. Elapsed: 0.002 sec.
```
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:
```
:) SELECT groupArray(y) FROM t_null_big
SELECT groupArray(y)
FROM t_null_big
┌─groupArray(y)─┐
│ [2,2,3] │
└───────────────┘
1 rows in set. Elapsed: 0.002 sec.
```
`groupArray` does not include `NULL` in the resulting array.