Re: OODB

"Scotty" <invallid_at_invalid.spam> schrieb im Newsbeitrag news:noaotu066nbjo304t5qbvrr74gfencntcv_at_4ax.com... > Do they exist or are they a figment of a college lecturers > imagination?

They do exist!

But, do REAL relational databases exist ?

Problem:

Consider the following families: Family Miller has kids John and Steve, Family Jones has kid Tom and Family Smith has no kids.

So you can design these two relations:

FAMILY (ID, NAME)
1 Miller
2 Jones
3 Smith

and KID(ID, NAME)
1 John
1 Steve
2 Tom

How do you get all the families with their kids ?

Solution a) the natural join:

select * from family f, kids k where f.id=k.id

gives you the result:

1 Miller 1 John
1 Miller 1 Steve
2 Jones 2 Tom

how can we interpret the result:

• Family Miller has 2 kids, because there are 2 rows with name Miller ?
• Family Jones has 1 kid, because there is 1 rows with name Jones ?
• Family Smith has no kids, because there is no row with name Smith ?

so far so good. But then, the following statements may also be true:

• Family Bush has no kids, because there is no row with name Bush ?
• or family Bush does not exist ?
• Family Smith is not a family, because they don't have kids ?

Solution b) ok, ok try a non relational workaround - the outer join: select * from family f left outer join kids k on f.id=k.id

1 Miller 1 John
1 Miller 1 Steve
2 Jones 2 Tom
3 Smith <NULL> <NULL>

At the first sight, the result looks slightly better:

• Family Miller has 2 kids, because there are 2 rows with name Miller
• Family Jones has 1 kid, because there is 1 rows with name Jones
• Family Smith exists.

but:

• Has Family Smith 1 kid, because there is 1 row with name Smith ?
• Is the statement 3 == <NULL> true or not ?
• Is the result a true relation ?

Solution c) ?

--
It looks like, that a "relational" database needs a non relational
operation, which, in return, gives you a invalid relation !
And as a result we have to live with these <NULL>s


Problem:

What is: "Relational Database" left outer join "Relational Algebra" ?

a) Relational <NULL>
b) <NULL> Database
c) both a) and b)
d) ?

Welcome to the real world ;-)

cheers,

Adrian.