Primary Key Vs. Unique Key in SQL server
| Primary Key | Unique Key |
| Primary key doesn’t allow Null values | Unique Key allows Null value. But only one Null value. |
| By default it create cluster Index | By default it create non cluster index. |
| Each table has only one primary key | Each table may have more than one unique key. The number of Unique constraints per table limited by no of non-cluster index per table. i.e. 249 for SQL server 2005 999 for SQL Server 2008 and later. |
| Logically Primary Key is a combination of Unique and NOT NULL Constraints. |
| Primary key and unique key, both support auto increment value. |
| Primary key and unique key can be made foreign key into another table. |