SQL SERVER – Difference Between Candidate Keys and Primary Key

Let us first try to grasp the definition of the two keys.

Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key.

Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.

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One needs to be very careful in selecting the Primary Key as an incorrect selection can adversely impact the database architect and future normalization. For a Candidate Key to qualify as a Primary Key, it should be Non-NULL and unique in any domain. I have observed quite often that Primary Keys are seldom changed. I would like to have your feedback on not changing a Primary Key.

I have illustrates the difference between a candidate key and a primary key in SQL Server.

1 Introduction
2 An Example to Understand Keys
2.1 Select a key that does not contain NULL
2.2 Select a key that is unique and does not repeat
2.3 Make sure that Primary Key does not keep changing
3 Selection of Primary Key
4 Quick Note on Other Kinds of Keys
5 Summary

Read Complete Article Here

Reference : Pinal Dave (https://blog.sqlauthority.com)

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18 Comments. Leave new

  • Can u plz explain what is difference between Composite Key and Candidate Key ?

    Also what is importance of Unique Key in Candidate key

    Reply
    • @Jasminder: A composite key is a primary key which has more than 1 column. For example consider the many to many relation between student and courses. Here we need another table , lets say studentcourses .

      This studentcourses table will have student id and course id as its columns. This will form your composite key.

      Candidate key is a set of columns which can uniquely identify the values in a table and can act as a unique key. One of these candidate keys will become the primary key and the rest will become alternate keys.

      Unique key as the name suggests , is used to uniquely identify a value in a table. For example Emp_id etc. Every candidate key will act as a unique key. Unique key can never be a NULL value.

      Reply
  • the main reason why it is called “composite key” is because it can only be made of two or more- but not one- attributes. a composite key is not just a primary key: it can be a candidate and superkey as well. but the original meaning lies in the fact that it is made up of more than one attribute

    Reply
  • how to identify condidate key plz tell me?

    Reply
  • avdhesh kumar
    March 22, 2012 2:24 pm

    how to create candidate key in a table .please give the example of candidate key

    Reply
  • Chris Stevenson
    May 30, 2012 7:46 pm

    If you have a table for countries in the world, with an id number, the name of the country, a two letter abbreviation of the country, and the country code for calling, and of those values could be a candidate key (they are all expected to be different from each other) but the most likely primary key would be the id number of the row.

    Reply
  • Hi can you please let me know process of Normalization with example……

    Reply
  • sateesh sasapu
    July 26, 2013 5:50 pm

    what is meant by view ? how to create and delete view ?

    Reply
    • A view(virtual table) is a temporary table that is formed out of one or more original tables.
      to create view
      create view
      as
      from
      where

      to delete
      delete

      Reply
  • superb explanation !

    Reply
  • please tell me how to use s_parse method in sql?

    Reply
  • Nice explanation for all answers. Plz tell me what is a super key with an example?

    Reply

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