Define primary key and candidate key.

Define primary key and candidate key.
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2 Answers

Answer :

Primary key: The PRIMARY KEY uniquely identifies each record in a database table. Primary keys must contain unique values. A primary key column cannot contain NULL values. Each table should have a primary key, and each table can have only one primary key. 

Candidate key: A minimal super key is called a candidate key. An entity set may have more than one candidate key. A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is special, and it is called the primary key
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Answer :

Primary Key: A primary key is an attribute in Relation that uniquely identifies the rows in relation. A Primary key does not hold NULL values and duplicate values. 

OR

A key which is selected by the designer to uniquely identify the entity is called as Primary key. A primary key cannot contain duplicate values and it can never contain null values inside it.

Example: In a Student table(Rollno,Name,Percentage), Rollno is the primary key

Candidate key: In a relation there may be a key or combination of keys which uniquely identify the record. Such a key is called as Candidate key. 

Example: Consider a Student table (Rollno,Name,Percentage), if (Rollno) and(Name)both are unique then both are identified as candidate keys. 

OR 

Consider a Student table (Rollno, Name, Percentage), if (Rollno, Name) is unique , then (Rollno, Name) can be a candidate key if and only if Name and Rollno individually are not unique. 
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