Difference between a primary key and a unique key in SQL?

In SQL both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a non-clustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

There can be more than one unique values for the columns in a table.

There is only one primary key for a column and primary key can be associated with other tables with foreign key relationship.

This is not possible with Unique key

There is a correction

There is only one primary key for a table.

primary key doesnot allow duplicate values and doesnot allow null.
unique key also doesnot allow duplicate values but allows one null value.

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There are more then one primary key in a table.Primary Key do not allow any null value.but unique key allow one null value.

when we compare both primary key and unique key,they appear to be similar but their functionalities may differ,bcoz primary key is to serve as identity b/w the two tables as unique key may not have.

Unique Key: Identify the row by uniquely, means if you are inserting a records into the table suppose you already inserted a record with ID 101 and another record you are inserting with the same ID then it will give an error(unique key voilation).

Prmary Key:Once you define a primay key on a particular column then a column become a mandatory field.That means you need to insert something on it, you can not leave as a blank otherwise it will give an error as primary key voilation.

what is actual defination of Dot Net??