Primary Keys & Foreign Keys

 Primary Keys & Foreign Keys


Primary Keys & Foreign Keys


What Is Primary Key?


⦁ The primary key is the unique identifier for the entire row, referring to one or more columns.

⦁ if there are more multiple columns for the primary key, then the set of primary key columns is known as a composite key.


Primary Keys & Foreign Keys




What Is Foreign Key?


⦁ A primary key in another (foreign) table.

⦁ Foreign keys are used to map relationships between tables.


Primary Keys & Foreign Keys


 

 Primary Key vs Foreign Key 


1 -

 Primary Key

⦁ A primary key is used to ensure data in the specific column is unique.

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


2 - 

 Primary Key

⦁ It uniquely identifies a record in the relational database table.

Foreign Key

⦁ It refers to the field in a table which is the primary key of another table.


3 - 

 Primary Key

⦁ Only one primary key is allowed in a table.

Foreign Key

⦁ Whereas more than one foreign key is allowed in a table.


4 -

Primary Key

⦁ It is a combination of UNIQUE and Not Null constraints.

Foreign Key

⦁ It can contain duplicate values and a table in a relational database.


5 - 

Primary Key

⦁ It does not allow NULL values.

Foreign Key

⦁ It can also contain NULL values.


6 - 

Primary Key

⦁ Its value cannot be deleted from the parent table.

Foreign Key

⦁ Its value can be deleted from the child table.


7 - 

Primary Key

⦁ Its constraint can be implicitly defined on the temporary tables.

Foreign Key

⦁ Its constraint cannot be defined on the local or global temporary tables.


Note : 

- The relational databases support many keys on It.

- Like (Super Key - Candidate Key - Composite Key - Surrogate Key - Alternate Key)


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