The PRIMARY KEY and UNIQUE KEY both are similar
and it used to apply the uniqueness of the columns and also used for uniquely identify each row in
database tables.
The major difference between Primary key, Unique
key and Foreign Key as given below.
Primary Key
1.
A primary key cannot accept null
values.
2.
A primary key must be unique.
3.
By default, primary key create only one clustered index in
the database .
4.
Each table can have one and only one
primary key.
5.
Each table can have one and only one
clustered index.
6.
Primary key can be made foreign key
into another table.
7.
PK support auto increments.
Unique
Key
1.
A unique key can accept only single
null value.
2.
A unique key does not have to be the
primary key.
3.
By default, unique key create non-clustered
index.
4.
Each table can have more than one
unique key.
5.
Unique key cannot made foreign key
into another table.
6.
UK not support auto increments.
Foreign
Key
1.
The foreign key is a field in the
table that is primary key in another table.
2.
The foreign key do not create
clustered or non clustered index automatically. You can create manually only.
3.
The foreign key can accept multiple
null values.
4.
You can have more than one foreign
key in a database table.
5.
There are actual advantages to
having a foreign key be supported with a clustered index, but you get only one
per table.
For example, you can seen in below pic for understanding about the same.
Reference links,
Reference links,
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![SQL Server "Primary Key", "Unique Key" and "Foreign Key" [Database]](https://4.bp.blogspot.com/-zZzlSnsPCQE/Vq8a73tJgaI/AAAAAAAAL3A/GbbjupwBMDA/s72-c/FK%2BUK%2BPK.png)