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Why use a Unique Key when you can use a Primary Key?

 
Greenhorn
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Hi, this is my first post, and I'm not sure if it should go in this topic or not, but here it goes. I was reading about unique keys and primary keys in Wikipedia, and it seems that a primary key is a special case of a unique key, in that a primary key cannot hold a NULL value.

I was wondering:
1) Why are unique keys called "unique keys"?
2) Why use a unique key when you can use a primary key?

Thanks in advance.
 
Ranch Hand
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A PK is considered to be an unique identifier of the row. It should never be subject to changes. For example the ID of the User.

An UK is considered to be unique thoroughout the whole column. It is not necessarily an identifier of the row as it may be subject to changes. For example the username or email address of the User.
 
Anderson Goldman
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Got it. Thank you!
 
Bauke Scholtz
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You're welcome.
 
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