shankcool

 Profile Answers by shankcool 

• Jun 2nd, 2008

 

A candidate key is a unique identifier that could be used as a primary key, which when you choose the primary key, the rest will then just be data fields

basabdutta9

 Profile Answers by basabdutta9 

• Oct 17th, 2008

 

Primary key is the unique key to identify a record and Candidate key is composed of one or more than one key to make it unique.

veerbahadur

 Profile Answers by veerbahadur 

• Dec 24th, 2008

 

Any attribute that is uniquely identify a row in a table is candidate key for the table. We select one of the candidate key as Primary key. All candidate keys which are not chosen as "primary key" are Alternate keys. The key which uniquely identify the rows of the table and which is made up of more than one attribute is called Composite key. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys.

Prabhat Sahu

 Profile Answers by Prabhat Sahu 

• Oct 12th, 2018

 

Key/SuperKey: It is the column or set of columns that can be use to uniquely identify the records/tuples in a table. There can be possibility of multiple SkeyuperKey in a relation/table.
Candidate key: The minimal SuperKey is considered as a candidate key.
There can be possibility of multiple candidatekey in a table.
Primary key: Among multiple candidate key Database Administrator can choose only one key as a Primary key.
Example: Suppose a table Rel1 is having multiple unique columns as below.
Rel1 (SSN, PAN, PASSPORTNO, Description)
DBAdministrator or DBArchitect can choose any unique column which do not have any NULL values in it as PK.
Unique key: It is the key column or combination of columns with a maximum of one NULL value in it.
In short- PK is having a column or combination of columns which is UNIQUE and NOT NULL. Where as a Unique key can have all UNIQUE values with maximum of a single NULL .