[Techtalk] Relational databases?

[jenn at simegen.com]

Kath wrote:

> What is it exactly?  I think I understand the concept, but I'm not sure.
> 
> Is it like an actual symlink or is it more like 

A relational database is an idea, not an implementation - according to 
my lecturers at Uni (lo, those many years back), even SQL is not
*totally* relational.

To paraphrase page 20 of the textbook (see, old textbooks are useful!)*:

A system is relational if:
* the data is perceived as tables and nothing but tables
* the operations at the users disposal ALSO create/modify tables, and 
nothing but tables. ('SELECT * FROM foo WHERE bar = "baz"' generates a 
mini-table)


SQL fits that definition - but there is a more precise definition later 
in the book. Non-relational databases roughly categorise (or did in the 
late 80's) as: heirarchic, inverted-list and network.


Summarising the introduction to chapter 15 in a few paragraphs:

The data must be constructed of single, atomic values, and of relationships
between those values.
Keys must be unique and non-null, foreign keys must perfectly match the
primary keys or be null.
Data manipulation must be done using relational algebra or relational 
calculus, and relational assignment. 



In the '80s, SQL didn't handle some of the more abstruse features of 
relational math - and no, I'm not even going to TRY summarising 
relational math in a single email!


I can pull other answers out of either my head or the book, if you 
have other questions. :)



Jenn V.
"In Introduction to Database Systems" 	C.J.Date	
(Volume 1, 4th edition....)
-- 
     "Do you ever wonder if there's a whole section of geek culture
             you miss out on by being a geek?" - Dancer.

jenn at simegen.com     Jenn Vesperman     http://www.simegen.com/~jenn/