Data bas testing basically include the following.

1)Data validity testing.

2)Data Integritity 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 referintial 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.