Index is the database table ordered by the value of one or more columns of a structure
Advantages:
1. To speed up the speed of data retrieval
2. To create a unique index, guarantee the uniqueness of each row of data in a database table
3. To speed up the connection between the table and table
4. In the use of grouping and sorting clause for data retrieval, can significantly reduce the query grouping and sorting time

Primary key is both constraints and index, the data is only allowed in the table has a primary key, but there can be multiple indexes,
Use a primary key, database will automatically create an index, the index is equivalent to a dictionary directories, can query to the desired result quickly, without the need for a full table scan,
The only index said index value only but can be null