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#data_type is array) and [Tuple](tuple.md). Composite data types can contain `Nullable` type values, such as `Array(Nullable(Int8))`.
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.