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What's the point of a candidate key?

Asked 2010-10-18T17:01:56.543
18

I'm fairly new to database management and this question never seems to be answered in more than one sentence. All other SO answers say "A candidate key is a minimal super key." That means nothing to me.

A candidate key is supposed to specify uniqueness of a db record, correct? And a primary key is a candidate key. If a primary key already specifies uniqueness, what's point of adding more candidate keys?

I have seen example records like the following:

Employee(ID, Name, PhoneNumber)

where ID is the primary key and PhoneNumber is a candidate key. From what I see, the ID is enough to specify the uniqueness of an employee record. Although PhoneNumbers are (probably) unique, specifying them as a candidate key does not seem "minimal" to me.

database database-design entity-relationship relational-database
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We use candidate key, because some times it is necessary to take care of the record and other issue, for example a data base for bank system, where Account_No is primary key and SocialSecurity_NO will be candidate key, we keep the SocialSecurity_NO as UNIQUE key, because if government got a mistake in providing the SocialSecurity_NO, then it will be big problem, so we have already declared it as UNIQUE key, so there is no possibility that two user with same SocialSecurity_NO will be able for account...

primary key----------- candidate key---------attribute3-------attribute4

Account_No SocialSecurity_NO

answered 2013-04-01T06:35:14.960

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