What is the difference b/w Primary Key and Unique Key

Question

I tried to find it out in google but not satisfactory answer is given out there. Can anybody explain the solid difference.

actually if Primary key is used to sele

Answer 1

Primary Key constraint
1. A primary key cannot allow null.
2. Multiple primary keys are NOT allowed.
3. On some RDBMS a primary key generates a clustered index by default.

Unique constraint
1. A unique constraint can be defined on columns that allow nulls.
2. Multiple unique keys are allowed.
3. On some RDBMS a unique key generates a nonclustered index by default.

Source Wikipedia