A table must contain only one primary key.if a table contains more than one primary key,then these are called composite keys.a primary key must be unique.but a unique key can't be a primary key.a table contains more than one unidue keys but a table must be identified by it's primary key. 

Primary key:
1)Primary key is nothing but it is uniqly identified each roe in Table.
2)Primary key Does not Allowes Duplicate values and Null values.
3)Primary key is default Clustered indexes
4)One table can have only one Primary key.
Unique Key:
1)Unique Key is nothing but it is uniqly identified each roe in Table.
2)Unique Key Does not Allowes Duplicate values but allowes only one Null value.
3)Primary key is default Non- Clustered indexes

Regards,

Narasima

• PRIMARY KEY: A tabLe can aontain only one PRIMARY KEY. It Doesn't allow null values, it is used  as Foreign Key In Another Table(like Reference Key).

• UNIQUE: A Table may Contain  one or more UNIQUE  constraints. Its aloow only one null value . Use this key We maintain unique vales in the table.

Both Primary and Unique Keys are enforce uniqueness of the column. But the differeces are given below:

Primary Key:

1) It creates clustered index by default

2) It doesn't allow nulls

Unique Key:

1) It creates non-clustered index by default

2) It allows only one null value

Regards

MadhuKumarKoppula

PRIMARY KEY Constraint

Specifies the column that uniquely identifies a row in the table. The identified columns must be defined as NOT NULL.

Note: If you attempt to add a primary key using ALTER TABLE and any of the columns included in the primary key contain null values, an error will be generated and the primary key will not be added. See ALTER TABLE statement for more information.

UNIQUE KEY Constraint

Specifies that values in the column must be unique. NULL values are not allowed.

Thanks!
Manoj [InfoAxon Technologies Ltd.]

A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys.

The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.