What is Database Testing?

For every project there will be a back end. Back end is very important to store large amount of data. In php projects generally we use mysql database. While testing data base we will test the fields.
For example while creating username field we will follow this

Field Name:Username

Form Field:Text Field

Validations:
6-20 characters limit.
Alpha numeric values allowed.
Only hyphen allowed among the specials characters.
Spaces not allowed.

While Testing we will generally enter more than 20 characters in the form filed username and check whether it is accepting.
We will check all the above values i.e Alphanumeric values(0-9,a-z,A-Z),-,spaces.
Generally we will do load testing on database.