What is a UNIQUE constraint in SQLite?

A UNIQUE constraint ensures no two rows in a table share the same value in that column (or combination of columns, for a multi-column unique constraint), raising a constraint violation error if you try to insert or update a row that would create a duplicate.

CREATE TABLE person (
    id INTEGER PRIMARY KEY,
    email TEXT UNIQUE
);

INSERT INTO person (email) VALUES ('ada@example.com');
INSERT INTO person (email) VALUES ('ada@example.com');   -- fails: UNIQUE constraint violation

Unlike PRIMARY KEY, a table can have multiple UNIQUE constraints across different columns, and unlike a primary key, a UNIQUE column can typically hold a NULL value (and SQLite treats multiple NULLs as not conflicting with each other under a unique constraint, since NULL is never considered equal to another NULL).

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