Difference Between Primary key and Unique key

Primary key and Unique key constraints are important feature in database system. They have some key identity which is important to know.

Unique Key constraints:
Unique key constraint will provide you a constraint like the column values should retain uniqueness.
It will allow null value in the column.
It will create non-clustered index by default
Any number of unique constraints can be added to a table.

Primary Key Constraint:
Primary key will create column data uniqueness in the table.
Primary key will create clustered index by default
Only one Primary key can be created for a table
Multiple columns can be consolidated to form a single primary key
It won't allow null values.

Learn GitHub - Introduction To Git

GitHub is a web-based hosting service for software development project that use the Git revision control system.
It will seems very difficult to co-operate with GitHub with novice people. But after using it, it will seems to you like magic. I hope you know why we will use GitHub!
If you are new to GitHub then below video will help you to start properly.
You can start from book also. There have several book but I will prefer you to start with this book Pro Git.
I hope this video and book will help you.