Nothing 

The only purpose of this data type is to represent cases where a value is not expected. So you can’t create a Nothing type value.

For example, literal NULL has type of Nullable(Nothing). See more about Nullable.

The Nothing type can also used to denote empty arrays:

SELECT toTypeName(array())

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