What is Database testing?
Database testing basically include the following.
1)Data validity testing.
2)Data Integrity testing
3)Performance related to data base.
4)Testing of Procedure,triggers and functions.
for doing data validity testing you should be good in SQL queries
For data integrity testing you should know about referential integrity and different constraint.
For performance related things you should have idea about the table structure and design.
for testing Procedure triggers and functions you should be able to understand the same.