---
title: Nullable Data Type reference
meta:
    description: Allows storage of NULL values alongside normal values.
---

# Nullable(T)

The `Nullable(T)` data type allows a column to store a special `NULL` marker, representing a missing or undefined value, in addition to the standard values of its base type `T`. For instance, a `Nullable(Int8)` column can hold `Int8` integers or `NULL`. Note that `T` cannot be a composite type like `Array`, `Map`, or `Tuple` directly, but these composite types can contain `Nullable` elements (e.g., `Array(Nullable(Int8))`). `Nullable` fields cannot be used in table indexes, and `NULL` is the default value for any `Nullable` type.

## Finding NULL

You can find `NULL` values in a column by using the `.null` subcolumn without reading the whole column. It returns `1` if the corresponding value is `NULL` and `0` otherwise.

### Example

```sql
SELECT
    value_col,
    value_col.null AS is_null_flag
FROM (
    SELECT CAST(1 AS Nullable(UInt32)) AS value_col
    UNION ALL
    SELECT CAST(NULL AS Nullable(UInt32)) AS value_col
    UNION ALL
    SELECT CAST(2 AS Nullable(UInt32)) AS value_col
    UNION ALL
    SELECT CAST(NULL AS Nullable(UInt32)) AS value_col
)
```

Result:

```result
┌─value_col─┬─is_null_flag─┐
│         1 │            0 │
│      ᴺᵁᴸᴸ │            1 │
│         2 │            0 │
│      ᴺᵁᴸᴸ │            1 │
└───────────┴──────────────┘
```

### General Usage Example

When performing operations with `Nullable` values, `NULL` typically propagates through expressions.

```sql
SELECT
    x,
    y,
    x + y AS sum_result
FROM (
    SELECT 1 AS x, CAST(NULL AS Nullable(Int8)) AS y
    UNION ALL
    SELECT 2 AS x, CAST(3 AS Nullable(Int8)) AS y
)
```

Result:

```result
┌─x─┬─y──┬─sum_result─┐
│ 1 │ ᴺᵁᴸᴸ │       ᴺᵁᴸᴸ │
│ 2 │  3 │          5 │
└───┴────┴────────────┘
```