Database / SQLite Interview questions
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).
What does a UNIQUE constraint enforce?
How does SQLite treat multiple NULL values in a UNIQUE column?
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