Answer
A UNIQUE constraint requires the constrained values to be distinct according to the database null rules. • A table can have several UNIQUE constraints. • A UNIQUE constraint can cover one or multiple columns. • Use PRIMARY KEY for the chosen main row identifier.
💡 SQL Example
CREATE TABLE users (user_id integer PRIMARY KEY, email text UNIQUE);
Result
CREATE TABLE
⚡ Quick Revision
UNIQUE prevents duplicate constrained values; PRIMARY KEY is the main row identifier.