Difference Between UNIQUE and PRIMARY KEY columns

What is the difference between UNIQUE and PRIMARY KEY columns?

With the primary key column we can identify rows of a table in a unique manner. This is very important when it comes to database programming. On the other hand, a unique key column,

• forbids having identical values on the field. For example, If you have column for storing names you can not have two same names. Data should be unique.
• You can have more than one unique key columns
• is allowed to accept NULL values. This is the differentiating factor between primary key and unique key. Primary key does not allow NULL values.

See the below example.

ID	Name	City
1	Rose	Chicago
2	Mike	San Antonio
3	Null	Chicago
4	Lian	Chicago

Here ID column is primary key and "Name" column is a unique key.