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

Just to add another example:

Think of a table holding user data, where each user has an email address. No two users can have the same email address, so that column becomes a unique key. While it could be the primary key (I have never made a string a primary key), it doesn't have to be.