keys

Primary key VS unique key vs foreign key

A primary key is a column of table which uniquely identifies each tuple (row) in that table.

Unique key constraints also identifies an individual tuple uniquely in a relation or table.

Comparison Chart:

PARAMENTER

PRIMARY KEY

UNIQUE KEY

Basic

Used to serve as a unique identifier for each row in a table.

Uniquely determines a row which isn’t primary key.

NULL value acceptance

Cannot accept NULL values.

Can accept one NULL value.

Number of keys that can be defined in the table

Only one primary key

More than one unique key

Index

Creates clustered index

Creates non-clustered index

Primary Key

Primary key cannot have a NULL value.
Each table can have only one primary key.
By default, Primary key is clustered index, and the data in database table is physically organized in the sequence of clustered index.
Primary key can be related to another tables as a Foreign Key.
We can generate ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
We can define Primary key constraint on temporary table and table variable.
We can't delete primary key value from the parent table which is used as a foreign key in child table. To delete we first need to delete that primary key value from the child table.

Unique Key

Unique Constraint may have a NULL value.
Each table can have more than one Unique Constraint.
By default, Unique key is a unique non-clustered index.
Unique Constraint can not be related with another table's as a Foreign Key.

Foreign Key

Foreign key is a field in the table that is Primary key in another table.
Foreign key can accept multiple null value.
Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
We can have more than one foreign key in a table.
Foreign keys do not automatically create an index, clustered or non-clustered. You must manually create an index on foreign keys.
There are actual advantages to having a foreign key be supported with a clustered index, but you get only one per table. What's the advantage? If you are selecting the parent plus all child records, you want the child records next to each other. This is easy to accomplish using a clustered index.
Having a null foreign key is usually a bad idea instead of NULL referred to as "orphan record".
We can’t define foreign key constraint on temporary table or table variable.
We can delete the foreign key value from the child table even though that refers to the primary key of the parent table.

PreviousJavaPersistence:HibernateandJPAFundamentals0 NextNoSQL DB

Last updated 5 years ago

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Primary Key

Primary key cannot have a NULL value.

Each table can have only one primary key.

By default, Primary key is clustered index, and the data in database table is physically organized in the sequence of clustered index.

Primary key can be related to another tables as a Foreign Key.

We can generate ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.

We can define Primary key constraint on temporary table and table variable.

We can't delete primary key value from the parent table which is used as a foreign key in child table. To delete we first need to delete that primary key value from the child table.

Foreign Key

Foreign key is a field in the table that is Primary key in another table.

Foreign key can accept multiple null value.

Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.

We can have more than one foreign key in a table.

Foreign keys do not automatically create an index, clustered or non-clustered. You must manually create an index on foreign keys.

There are actual advantages to having a foreign key be supported with a clustered index, but you get only one per table. What's the advantage? If you are selecting the parent plus all child records, you want the child records next to each other. This is easy to accomplish using a clustered index.

Having a null foreign key is usually a bad idea instead of NULL referred to as "orphan record".

We can’t define foreign key constraint on temporary table or table variable.

We can delete the foreign key value from the child table even though that refers to the primary key of the parent table.