Difference Between Primary Key and Foreign Key (With Table)

Primary Key vs Foreign Key

Database management system (DBMS) is important in areas like business, banking, airlines, telecommunication, and universities because it is an efficient and reliable method when it comes to creating, storing, and managing data in databases. Relational Database Management System (RDBMS) is the advanced form of DMBS structured particularly for relational databases. These databases preserve data in tables that contain rows and columns that symbolize an entry and an attribute respectively.

Keys are vital components of the relational database schema because they establish a link between two different tables and makes sure to uniquely identify any row of data within a table. However, keys have a more specific importance than just maintaining links. It helps in collecting specific data from numerous rows which would’ve been intricate or even impossible at times if it weren’t for keys. The two basic keys that help establish and identify links between relational databases efficiently and they are Primary keys and Foreign Keys.

Without the primary key, a table cannot count as a relational database table. Because primary keys are used to uniquely identify a specific row of data, two or more rows can’t have similar primary keys. Contrastingly, foreign keys are used to establish a relationship between the records of two distinct tables. Unlike primary keys, however, we can have numerous foreign keys in a database.

The difference between primary key and foreign key is that primary key uses the data contained within one or many columns in the table to identify each row uniquely whereas a foreign key is a column or multiple columns in a database that points to the primary key of a database record in some other table.


 

Comparison Table Between Primary Key and Foreign Key (in Tabular Form)

Parameter of comparisonPrimary keyForeign key
Duplicate valuesPrimary keys do not permit two rows to have the same values.Foreign keys permit the two rows to have the same values.
InsertionIn primary keys, one can insert values even if the foreign key does not have that value.In foreign keys, one cannot insert values if the values are not present in the primary key.
RangeEach row in the relational database table can have only one primary key.A relational database table can have numerous foreign keys.
Clustered indexPrimary keys, by default, have a clustered index.Foreign keys do not have a clustered index.
DeletionWhen a value is to be deleted, one should make the value is still not present in the foreign keys referencing table.When a value is to be deleted one can do it from foreign keys with ease.
Temporary tablesPrimary keys can be defined on a temporary table.Foreign keys cannot be defined on a temporary table.

 

What is Primary Key?

A primary key refers to a particular choice of columns that uniquely determines the set of rows in a table. A primary key is a single attribute that has a unique ID and is also considered as a candidate key. There are two types of primary keys, i.e., a simple primary key and a composite primary key. A simple primary key is a database table made of only one column whereas a composite primary key in a database table with more than one column.

The value of a primary key should never fluctuate or be changed because the primary key characterizes an importance in the database. So by changing the value it would result in a lot of confusion. The primary key shouldn’t contain a null value in the beginning so that no one can identify the value in that table. Additionally, no two rows will have the same primary key as this would not result in the database being unique. Hence, there can be only one primary key for every row of data in a table.

The Primary key is cluster-indexed which means all the rows in the table are sorted. The primary key is mainly defined on a temporary table. When one is deleting rows from a table they must be careful that the deleted value is not present in the foreign key column.

Primary Key
 

What is Foreign Key?

A foreign key refers to a column in a database table that provides a link between two tables. When table 1 has a primary key of table 2, then this attribute is the foreign key. The table 1 having the foreign key is called referencing key and the primary key of table 2 is called the referenced relation.

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Unlike primary keys, foreign keys can contain null values as it does not help in identifying a difference in the relation because primary keys have already done this part of the job. Foreign keys can also accept duplicate values unlike the primary key and can have many or multiple foreign keys in a database as foreign keys can have different attributes. However, foreign keys cannot have a clustered-index.

Foreign keys cannot be defined on a temporary table. While adding a value into the foreign key column, one must make sure the value is present in the primary key and there is no constraint when deleting values from a foreign key. Foreign tables help in maintaining the integrity of tables in the database. An accidental deletion of any value can be prevented by the database management system.

Foreign Key

Main Differences Between Primary Key and Foreign Key

  • Primary keys do not allow two rows in a database table to have duplicate values. Foreign keys allow two rows in a database table to have duplicate values.
  • In the attribute of the primary keys, values can be inserted in the table even if the foreign key doesn’t have that value in its column. In the foreign keys attribute, values cannot be inserted in the table if the values are not present in the primary key database.
  • A primary key attribute can have only one range of primary keys in its table. A foreign key attribute can have a wide range of foreign keys in its table.
  • Primary keys have a clustered index which means that all the rows in the database relation are sorted. Foreign keys do not have automatic clustered-index but it can be done manually.
  • Under primary keys, a value can be deleted from the referencing table by making sure that the value is not present in the foreign key reference table. Under foreign keys, the values can be deleted without any glitches since it does not matter whether the value is still present in the primary keys reference table or not.
  • Primary keys can be determined on a temporary table. Foreign keys cannot be determined or enforced on a temporary table.

 

Conclusion

In the database management system, keys play an important role in establishing relationships within a table and among different tables as well. But to do so, we must make sure that the areas we use to maintain relationships between different tables must have comparable values and the table must comprise of unique rows.

This article talks about the two most essential and common keys that create links among tables and help the relational database management systems work efficiently. Primary keys and foreign keys are described in detail and differentiated to help individuals clearly understand how different both are from one another because even though these terms might be similar in design, they perform diverse roles.

A primary key uses the data within one column or multiple columns to uniquely identify all rows in a relational database table to avoid inserting a duplicate row of data whereas a foreign key is used to establish a link between two tables in a database.


 

Word Cloud for Difference Between Primary Key and Foreign Key

The following is a collection of the most used terms in this article on Primary Key and Foreign Key. This should help in recalling related terms as used in this article at a later stage for you.

Difference between primary key and foreign key