ClickHouse/docs/en/query_language/agg_functions/index.md

<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.