[GRLUG] Database query logic

[Eric Beversluis]

I've been doing some reading on The Raiser's Edge. I found it
interesting  that, when explaining queries, they emphasized that the
queries do "filtering." This raised the question for me whether
relational DB queries actually "filter" or whether the RE talk of
"filtering" is heuristic.

In other words, which of these two descriptions fits how a database
query works?
1. The "selection" method: If I say "select FName from tblPeople," the
programs opens a file and "copies" all the FName values to a new file
(or into memory). 
2.  The "filter" method: If I say "select FName from tblPeople," the
program opens a file, deletes everything that is not "FName," and saves
the result to a new file (or into memory). 

It seems to me that these two approaches are in fact different and would
be programmed very differently.

I've never thought of DBs as using the "filter" approach, but then I
haven't thought very hard about it either.

Are these in fact two different logical approaches? If so, which do
relational DBs use--or do some use one approach and some the other?


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