Support NULL and NOT NULL Modifiers in CREATE Query

Why it matters

This feature allows users to explicitly define whether a column can store NULL values or not, improving schema clarity and enabling better data validation and integrity enforcement.

How to use it

When creating a table, specify the nullability of columns using the NULL or NOT NULL modifiers in the CREATE TABLE statement, for example:

CREATE TABLE example (
    id UInt64 NOT NULL,
    name String NULL
) ENGINE = MergeTree()
ORDER BY id