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what is candidate key?

Asked by: Interview Candidate | Asked on: Oct 4th, 2005
Showing Answers 1 - 8 of 8 Answers
Biswa Ranjan Das

Answered On : Oct 4th, 2005

A key that uniquely identifies rows in a table. Any of the identified candidate keys can be used as the table's primary key. Any of the candidate keys that is not part of the primary key is called an alternate key. One can describe a Candidate Key as a Super Key that contains only the minimum number of columns necessary to determine uniqueness.The importance of candidate keys is that they tell us how we can identify individual tuples in instances of a relation. As such they are one of the most important types of database constraints that should be specified when designing a database schema. Since an instance of a relation is always a set it holds that every relation will have at least one candidate key. Since in some RDBMSs tables may also represent multisets (which strictly means these DBMSs are not relational) it is an important design-rule to specify explicitly at least one candidate key for each relation. For practical reasons RDBMSs usually require that for each relation one of its candidate keys is declared as the primary key, which means that it is considered as the preferred way to identify individual tuples. Foreign keys, for example, are usually required to arrive in such a primary key and not in any other of the candidate keys.

  
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Biswa Ranjan Das

Answered On : Oct 4th, 2005

A key that uniquely identifies rows in a table. Any of the identified candidate keys can be used as the table's primary key. Any of the candidate keys that is not part of the primary key is called an alternate key. One can describe a Candidate Key as a Super Key that contains only the minimum number of columns necessary to determine uniqueness.

  
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ahsan

Answered On : Nov 14th, 2005

View all answers by ahsan

foreign key, also called a foreign keyword, in a database table is a key from another table that refers to (or targets) a specific key, usually the primary key , in the table being used. A primary key can be targeted by multiple foreign keys from other tables. But a primary key does not necessarily have to be the target of any foreign keys.

  
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ahsan

Answered On : Nov 14th, 2005

View all answers by ahsan

A candidate key is a combination of attributes that can be uniquely used to identify a database record without any extraneous data. Each table may have one or more candidate keys. One of these candidate keys is selected as the table primary key.

  
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harish

Answered On : Jan 2nd, 2006

a candidate key is a column in a table which has the ability to become a primary key. note: all alternate cannot be a candidate key

  
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Shilpa

Answered On : Jan 9th, 2006

The candidate key must be unique within its domain.The candidate key can not hold NULL values.The candidate key can never change. It must hold the same value for a given occurrence of an entity for the lifetime of that entity.Eg: we say that the main purpose of the candidate key is to help us to identify one single row in a table, regardless of whether there exists billions of row. This sets high demands on the flexibility in terms of delivering uniqueness.

  
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sandeep kumar

Answered On : Jan 16th, 2006

key is the attribute that can uniquely identify specified record from  a table. Any key i.e. attribute or combination of attributes can be selected as primary key. the remaining key attributes are called as candidate keys or alternate keys. these can also be use in place of the primary key. For example, let we consider a table student with attributes <rollno, accession_no,name,...> here rollno and accession_no both can be use as primary key. the remainin key attribute is candidate key.

  
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lax

Answered On : Aug 22nd, 2006

A candidate key is a key that uniquly identifies

  
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