ClickHouse/docs/en/data_types/special_data_types/nothing.md

Nothing

The only purpose of this data type is to represent NULL, i.e., no value.

You can't create a Nothing type value, because it is used where a value is not expected. For example, NULL is written as Nullable(Nothing) (Nullable is the data type that allows storing NULL in tables).

The Nothing type is also used to denote empty arrays:

:) SELECT toTypeName(Array())

SELECT toTypeName([])

┌─toTypeName(array())─┐
│ Array(Nothing)      │
└─────────────────────┘

1 rows in set. Elapsed: 0.062 sec.