Primary Key - SQL
A PRIMARY KEY specification carries an implicit NOT NULL constraint on each column of the specified key. When more than one key constraint is required, the key word UNIQUE must be used in place of PRIMARY KEY for all or all but one of them. A UNIQUE specification does not carry an implicit NOT NULL constraint on each column of the specified key (says the SQL standard, though I am aware of at least one SQL implementation where it does).
Whether declared using PRIMARY KEY or UNIQUE, at least one column must be specified.