Primary key vs unique

[Alexander Vasilevsky]

Give What is the difference in the primary key from unique?

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[Teemu Keiski]

Primary key is always unique while unique key isn't necessarily always a
primary key (although it quite often is). One difference is the NOT NULL
constraint (not enforced for UK while it is enforced for PKs) and that table
can have only one primary key but many unique keys.

http://en.wikipedia.org/wiki/Primary_key says it well