In testing process DB testing is necessary to know the
impact of front end operations on back end tables.
In Data Base testing testers are concentrating on data
validation and data integrity.

Data validation means validating insertion of new values
into   database  correct or not. 

Data integrity means validating the changes in existing
database correct or not, due to insertion of new values.

Frequently required quarries are select,update,delete,insert.

DB testing

Frequently used queries: Select,insert,delete,update
apart from that one may need to write simple and vomplex 
sub queries.
ineer joins
outer joins
variation in select statement
execute a stored procedure - exec <stored proc name>
Date functions which are one of the important variable in 
any database