Answer: NOT NULL

According to Snowflake's official documentation and the community discussion with high upvotes, Snowflake supports defining various ANSI SQL constraints (UNIQUE, PRIMARY KEY, FOREIGN KEY, NOT NULL) but only enforces NOT NULL constraints. The other constraint types are defined but not enforced by Snowflake's query engine. This is explicitly stated in Snowflake's constraints documentation and confirmed by multiple community members with upvoted responses.

Author: LeetQuiz Editorial Team