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Nullable ​

Nullable(T) allows a missing value where a typed member would otherwise require a value of type T. It is particularly useful for elements of arrays, tuple fields and map values.

Syntax ​

text
Nullable(T)

Column and member nullability ​

Top-level SQL columns permit NULL unless they declare NOT NULL, including columns whose type is written without Nullable. The wrapper is needed to admit NULL inside a native composite value.

For example, Array(UInt8) requires a non-NULL integer at every position, while Array(Nullable(UInt8)) can contain missing elements. Map keys cannot be nullable.

Nullable can wrap supported collections as well as scalar types. Use a column NOT NULL constraint to forbid a missing value at the top level.

Finding NULL ​

Most comparisons with NULL return NULL, rather than true or false. Use IS NULL or IS NOT NULL to test for a missing value. Use coalesce to supply a fallback. Aggregate functions document whether they skip or retain NULL values.

Usage Example ​

A nullable array element ​

sql
SELECT CAST('[1,null,3]' AS Array(Nullable(UInt8)))
FORMAT csv;
text
"[1,null,3]"

Testing and replacing NULL ​

sql
SELECT NULL = NULL,
       NULL IS NULL,
       coalesce(NULL, 7)
FORMAT csv;
text
\N,1,7

See NULL functions for additional operations.