ClickHouse/docs/en/data_types/nullable.md

# Nullable(TypeName) {#data_type-nullable}

Allows to store special marker ([NULL](../query_language/syntax.md)) that denotes "missing value" alongside normal values allowed by `TypeName`. For example, a `Nullable(Int8)` type column can store `Int8` type values, and the rows that don't have a value will store `NULL`.

For a `TypeName`, you can't use composite data types [Array](array.md) and [Tuple](tuple.md). Composite data types can contain `Nullable` type values, such as `Array(Nullable(Int8))`.

A `Nullable` type field can't be included in table indexes.

`NULL` is the default value for any `Nullable` type, unless specified otherwise in the ClickHouse server configuration.

## Storage features

To store `Nullable` type values in table column, ClickHouse uses a separate file with `NULL` masks in addition to normal file with values. Entries in masks file allow ClickHouse to distinguish between `NULL` and default value of corresponding data type for each table row. Because of additional file, `Nullable` column consumes additional storage space compared to similar normal one.

!!! info "Note"
    Using `Nullable` almost always negatively affects performance, keep this in mind when designing your databases.

## Usage example

```sql
CREATE TABLE t_null(x Int8, y Nullable(Int8)) ENGINE TinyLog
```
```sql
INSERT INTO t_null VALUES (1, NULL), (2, 3)
```
```sql
SELECT x + y FROM t_null
```
```text
┌─plus(x, y)─┐
│       ᴺᵁᴸᴸ │
│          5 │
└────────────┘
```

[Original article](https://clickhouse.yandex/docs/en/data_types/nullable/) <!--hide-->