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What Is SQL Candidate Key? Difference between Primary Key & Candidate Key

Keys are defined as a set of attributes that are generally needed to access tuples from a table. Keys are also used to construct relationships between the two tables. In this blog, we will discuss what is the primary key and the candidate key and significant differences between them. Both keys are responsible to identify a tuple within a table or a relation. The major difference is that there can be only one primary for a table. However, candidate keys can be multiple in a database relation. Let us learn the definition of both keys in brief below.

  • Candidate Key – A candidate key is a column or a set of columns that can qualify as a primary key in the database. There can be multiple candidate keys in a database relation and each candidate can work as a primary key for the table.
  • Primary Key – A primary key is a column or a set of columns that identifies some record uniquely. Only one candidate key can qualify as the primary key.

You have to be extra careful when deciding on the candidate or primary key. The incorrect selection may adversely impact the database structure and future normalization. To qualify the candidate key as the primary key, it should be Not Null and unique in the domain. When you start working on databases, you will realize that primary keys are seldom changed. SQL Server Curriculum Here are the few points to consider when converting a candidate key to the primary key:

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  • Make sure that it does not contain any NULL value.
  • Make sure that it is unique and does not repeat.
  • Make sure that the primary key is not changed frequently.

Other than the primary key and the candidate key, the next important concept to focus is a foreign key. A foreign key in the database is the key from another table that refers to the primary key in the table being used.

Primary key vs Candidate key – Basic Comparison Chart

Parameters for Comparison Primary Key Candidate Key
Basic There can be only one primary key in a relation They can be multiple candidate keys in a relation.
Specify It is not mandatory to specify a primary key for each relation. It is not possible to define any relation without the candidate key.
Null No attributes of the primary key can contain Null values. The attributes of the candidate key can contain Null values.
Feature A primary key defines the most important attribute of a relation. Candidate key features candidates that can be defined as the primary key.
Vice-versa A primary key is the candidate key. Each candidate key is not the primary key.

What is a Primary Key?

A primary key is an attribute or a set of attributes that uniquely identify each tuple in a relation. There is one primary key for each relation within a database. Keep in mind that the primary key should never contain NULL values. In case, one column contains Null values then it cannot be defined as the primary key. Further, the values of attributes of the primary key need to be static and the value of attributes changes rarely. SQL Server quiz One of the candidate keys gets qualified to become a primary key. The rules that are followed by the candidate key to qualify as the primary key include Null and unique parameters. If a relation contains an attribute that is defined as the primary key of some other relation then this attribute is called the foreign key. It is always advised to figure out the primary key of a database relation before you add other attributes to a relation as primary key identifies each tuple uniquely. It is always good to choose either a single attribute or a small number of attributes as the primary key that makes the relation handling easy. Let us understand the concept in deep with the help of an example below. Student {ID, First_name, Last_name, Age, Address} First, find out the possible number of candidate keys for this relation. In this example, there are three candidate keys ID, First-name, Last-name. Out of these three, ID can be defined as the primary key because there are chances that two students share the same first and the last name but ID will remain unique in such cases.

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What are the features?

  • Primary key enforces the maximum integrity of the table.
  • It always contains unique data only.
  • The length of a primary key cannot exceed more than 900 bytes.
  • A primary key can never contain the Null values.
  • There should not be any duplicate value for the primary key.
  • A table can contain only one primary constraint
  • When we define the primary key constraint for the table, the database engine automatically creates unique index for the primary key column.
  • In Oracle, it is not allowed for the primary key to include more than 32 columns.
  • For a database programmer, it is a common practice to create a primary key, modify it, and remove it. To remove a primary key, you should use the drop keyword.

When you are working on complex database designs, a deep understanding of different SQL keys is necessary that includes a primary key, foreign key, candidate key, composite key, super key, unique key and more. There is a slight difference between them but you should clear with the differences otherwise it may lead to serious confusion later.

What is a Candidate Key?

A candidate key is an attribute or a set of attributes that uniquely defines the tuple within a relation. As we know, there is more than one candidate key in a relation. Out of all candidate keys, one candidate can qualify to become a primary key. Though each candidate key qualifies to become a primary key, only one should be chosen as the primary key. The rules that a candidate key requires to become the primary key are the attribute values of the key can never be Null for any domain and it must be unique or static. If all candidate keys qualify to become a primary key then experienced DBA should take a decision to figure out the primary key. There is no relation possible to establish without a candidate key. free SQL Server demo Let us understand the candidate key with an example. If we add some more attributes to Student relation, I discussed above. Student {ID, First_name, Last_name, Age, Address, DOB, Department_name} Here, we may identify two candidate keys that are ID and DOB. It will help you to understand the candidate keys are the one that uniquely identifies a tuple in a relation.

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Primary key vs Candidate Key – What are the key differences?

  • The basic point of differentiation here is that there can be only one primary key in the schema and multiple candidate keys in a relation within the database.
  • The attributes under primary key can never be NULL as the major function of the primary key is to identify records in a relation uniquely. Even a primary key can be used as the foreign key in other relation, so it must not be Null so that it may reference relation to find tuples in a referenced relation. However, the candidate key can be Null unless the attribute constraint is specified not null.
  • It is not mandatory to specify a primary key but there cannot be a relation without candidate key.
  • Primary key describes the unique and most important attribute of a relation whereas candidate key provides candidates among which one can be taken as the primary key.
  • Every primary key is a candidate key but every candidate key is not the primary key. Candidate keys are frequently referred to as primary keys, secondary keys, and alternate keys too.

Points to Remember:

  • A column is named as the primary key that uniquely identifies each row within a table.
  • To create a primary key in SQL, you should use the Primary key constraint to create or modify a table.
  • When you can use multiple columns together as the primary key then it is named as the composite primary key.
  • To design a composite primary key, you should use as few columns as possible. It is good for storage and performance both, the more columns you add as the primary key, more storage space you require.
  • In terms of performance, limited data means database can process much faster than your imagination.

Conclusion: You have to be extra cautious when defining the primary key and the candidate key for a relation within the database. All candidate keys can be the candidate for the primary key and DBA should take a decision here about the primary key based on certain rules that need to be followed to qualify as the primary key. Keep in mind that always choose the right primary key which is the very basic step in the database design. To know more on different keys, you should join SQL certification program at the JanBask training learn to use these keys practically.

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