[Database class] Advice on query testing

[Paul S. Wang]

Dear DB class,

It is important to note that a successful query 
producing a nonempty result set is no  proof that you have the correct
query.  

The query may simply work correctly for the database instance
you used to test it, or the resultset is missing records that ought to be
included, or the resultset includes records that ought to be excluded,
or the query may break if a table becomes empty or certain attriutes
are \verb+erb+NULL+.  A developer must examine the logic of the query
carefully, avoid any assumptions of values in the database, and test
each query extensively.

Paul