> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-mintlify-3a82795f.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

> Documentation for the Array data type in ClickHouse

# Array(T)

An array of `T`-type items, with the starting array index as 1. `T` can be any data type, including an array.

<h2 id="creating-an-array">
  Creating an Array
</h2>

You can use a function to create an array:

```sql theme={null}
array(T)
```

You can also use `[]`.

```sql theme={null}
[]
```

Example of creating an array:

```sql theme={null}
SELECT array(1, 2) AS x, toTypeName(x)
```

```text theme={null}
┌─x─────┬─toTypeName(array(1, 2))─┐
│ [1,2] │ Array(UInt8)            │
└───────┴─────────────────────────┘
```

```sql theme={null}
SELECT [1, 2] AS x, toTypeName(x)
```

```text theme={null}
┌─x─────┬─toTypeName([1, 2])─┐
│ [1,2] │ Array(UInt8)       │
└───────┴────────────────────┘
```

<h2 id="working-with-data-types">
  Working with Data Types
</h2>

When creating an array on the fly, ClickHouse automatically defines the argument type as the narrowest data type that can store all the listed arguments. If there are any [Nullable](/reference/data-types/nullable) or literal [NULL](/reference/settings/formats#input_format_null_as_default) values, the type of an array element also becomes [Nullable](/reference/data-types/nullable).

If ClickHouse couldn't determine the data type, it generates an exception. For instance, this happens when trying to create an array with strings and numbers simultaneously (`SELECT array(1, 'a')`).

Examples of automatic data type detection:

```sql theme={null}
SELECT array(1, 2, NULL) AS x, toTypeName(x)
```

```text theme={null}
┌─x──────────┬─toTypeName(array(1, 2, NULL))─┐
│ [1,2,NULL] │ Array(Nullable(UInt8))        │
└────────────┴───────────────────────────────┘
```

If you try to create an array of incompatible data types, ClickHouse throws an exception:

```sql theme={null}
SELECT array(1, 'a')
```

```text theme={null}
Received exception from server (version 1.1.54388):
Code: 386. DB::Exception: Received from localhost:9000, 127.0.0.1. DB::Exception: There is no supertype for types UInt8, String because some of them are String/FixedString and some of them are not.
```

<h2 id="array-size">
  Array Size
</h2>

It is possible to find the size of an array by using the `size0` subcolumn without reading the whole column. For multi-dimensional arrays you can use `sizeN-1`, where `N` is the wanted dimension.

**Example**

```sql title="Query" theme={null}
CREATE TABLE t_arr (`arr` Array(Array(Array(UInt32)))) ENGINE = MergeTree ORDER BY tuple();

INSERT INTO t_arr VALUES ([[[12, 13, 0, 1],[12]]]);

SELECT arr.size0, arr.size1, arr.size2 FROM t_arr;
```

```text title="Response" theme={null}
┌─arr.size0─┬─arr.size1─┬─arr.size2─┐
│         1 │ [2]       │ [[4,1]]   │
└───────────┴───────────┴───────────┘
```

<h2 id="reading-nested-subcolumns-from-array">
  Reading nested subcolumns from Array
</h2>

If nested type `T` inside `Array` has subcolumns (for example, if it's a [named tuple](/reference/data-types/tuple)), you can read its subcolumns from an `Array(T)` type with the same subcolumn names. The type of a subcolumn will be `Array` of the type of original subcolumn.

**Example**

```sql theme={null}
CREATE TABLE t_arr (arr Array(Tuple(field1 UInt32, field2 String))) ENGINE = MergeTree ORDER BY tuple();
INSERT INTO t_arr VALUES ([(1, 'Hello'), (2, 'World')]), ([(3, 'This'), (4, 'is'), (5, 'subcolumn')]);
SELECT arr.field1, toTypeName(arr.field1), arr.field2, toTypeName(arr.field2) from t_arr;
```

```test theme={null}
┌─arr.field1─┬─toTypeName(arr.field1)─┬─arr.field2────────────────┬─toTypeName(arr.field2)─┐
│ [1,2]      │ Array(UInt32)          │ ['Hello','World']         │ Array(String)          │
│ [3,4,5]    │ Array(UInt32)          │ ['This','is','subcolumn'] │ Array(String)          │
└────────────┴────────────────────────┴───────────────────────────┴────────────────────────┘
```