Answer
A UNIQUE constraint ensures all values in a column or column group are distinct.
Quick Answer
UNIQUE constraint ensures all values in a column (or combination of columns) are distinct. Unlike PRIMARY KEY, UNIQUE allows one NULL value. PostgreSQL automatically creates a unique index to enforce it. Add to single column: email TEXT UNIQUE. Composite unique: UNIQUE(user_id, product_id). Use to enforce natural business keys (email addresses, usernames, product codes).
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SugharaIQ Editorial Team
Verified Answer
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Source: SugharaIQ