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Difference between Primary Key and Foreign Key
  • Difficulty Level : Basic
  • Last Updated : 28 Mar, 2020

Primary Key:
A primary key is used to ensure data in the specific column is unique. It is a column cannot have NULL values. It is either an existing table column or a column that is specifically generated by the database according to a defined sequence.

Example: Refer the figure –
STUD_NO, as well as STUD_PHONE both, are candidate keys for relation STUDENT but STUD_NO can be chosen as the primary key (only one out of many candidate keys).

Foreign Key:
A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It is a column (or columns) that references a column (most often the primary key) of another table.

Example: Refer the figure –
STUD_NO in STUDENT_COURSE is a foreign key to STUD_NO in STUDENT relation.

Figure:



Let’s see the difference between Primary Key and Foreign Key:

S.NO.PRIMARY KEYFOREIGN KEY
1A primary key is used to ensure data in the specific column is unique.A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables.
2It uniquely identifies a record in the relational database table.It refers to the field in a table which is the primary key of another table.
3Only one primary key is allowed in a table.Whereas more than one foreign key are allowed in a table.
4It is a combination of UNIQUE and Not Null constraints.It can contain duplicate values and a table in a relational database.
5It does not allow NULL values.It can also contain NULL values.
6Its value cannot be deleted from the parent table.Its value can be deleted from the child table.
7It constraint can be implicitly defined on the temporary tables.It constraint cannot be defined on the local or global temporary tables.

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