Answer Posted / madhumita majhi

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.

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.

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