Constraints

A constraint is a rule the database enforces, so bad data cannot get in even by accident.

NOT NULL, UNIQUE, PRIMARY KEY, REFERENCES, CHECK, DEFAULT.

Validation in your application produces a helpful error message. The constraint is what makes the rule true, regardless of which application, script, migration or person is writing at the time.

Each of those three violates a different constraint. Uncomment one at a time and read what SQLite says.

✦ SQLite does not enforce foreign keys by default

It parses REFERENCES and then, unless you run PRAGMA foreign_keys = ON, does not check it. That surprises people moving from SQLite to PostgreSQL, where a foreign key that was quietly ignored in development suddenly rejects data in production.