5.5 SELECT Phrases
Queries must be written based on the names and headings of the tabular variables and not on the tables that represent their values at any given moment. This is similar to writing programs. A program should work for all legal inputs and not just the ones on which it was tested. In both cases, it is important to
78 SQL — The Relational Language
focus on the abstract structure and not on specific examples. The way we write SQL constructs must be directed only by the logic of the query and not by the content of a particular database instance. Just because the query generated the right answer for a particular instance of the database does not mean that it is correct.